Embedded Turnouts for the Hiawatha LRT Project · and Hong Kong. These “flexive tongue” designs are ideal for paved track applications and can provide maintenance advantages

Embedded Turnouts for the Hiawatha LRT Project

Author: David F. Peterson, PE

Parsons Corporation

Design Manager, Hiawatha Light Rail Transit Project

September 30, 2003

ABSTRACT

Installation and maintenance of turnouts in paved areas have always been

something to avoid for both railroads and transit agencies. For the development

of the Hiawatha Light Rail Transit Project, a key element is at-grade rail access

to downtown Minneapolis. The present north end in downtown requires

crossovers in Fifth Street and Nicollet Mall, a confined area congested with

utilities and vehicular traffic. Embedded turnouts – turnouts installed in paving –

are required in this and other locations along the Hiawatha corridor.

Minnesota Transit Constructors (MnTC), the design-build contractor for the

Hiawatha Project Office, elected to use #6 flexive double tongue switches for

paved areas. This European Edgar Allen switch design differs from AREMA

standard double tongue turnouts by having a flexible switch point set in a fixed

casting, a transit – type design also being used in Tampa’s new LRT system.

For containment of stray electrical current to prevent corrosion of nearby

infrastructure, the embedded track is electrically isolated. The turnout is

constructed in a concrete “tub” comprised of concrete and insulating membrane

and sealed with elastomeric grout. The turnout and tub are constructed with

three concrete pours, with the insulating membrane installed between the first

and second pours.

In most cases, the turnouts are power operated with switch machines between

the switch point castings, and heated with calrod-type electrical resistance

heaters.

The turnouts were fabricated by Progress Rail Services and will be installed by

MnTC’s subcontractor Railworks Track Systems in early to mid 2003.

1. INTRODUCTION

Construction and operation of turnouts in paved areas have always been a less-

than-desired alternative to ballasted designs. These “embedded” turnouts

require closer construction tolerances and higher maintenance and are avoided

when possible.

The Hiawatha Light Rail Transit project in Minneapolis and Bloomington,

Minnesota is currently under construction. Hiawatha is a new system linking

downtown Minneapolis with the Minneapolis – St. Paul International Airport and

the Mall of America in Bloomington. The line is approximately 12 miles in length

and includes seventeen stations located in commercial and residential areas.

Low floor Bombardier light rail vehicles will operate at up to 55 mph at 7 ½

minute headways at peak travel periods. The system requires six embedded

crossovers, primarily at the end points of the system to cross trains over for

return movements.

The design-build contractor, Minnesota Transit Constructors (MnTC), worked

closely with the representatives of the Hiawatha Project Office (HPO) to identify

the most appropriate embedded track and turnout system for the Hiawatha LRT

system.

2. TURNOUT EVALUATION

The HPO’s two agencies – the Minnesota Department of Transportation and

Metro Transit, decided during the design development process to use tee rail in

lieu of girder rail for embedded track applications. This in turn led to discussions

of alternative types of turnouts.

AREMA’s Portfolio has two embedded turnout types that in the past have been

used both for railroad and light rail transit applications. (See Portfolio Plan No.

980-60 and other referenced plans). These turnouts, using tongue and mate

and double tongue switches, would have performed satisfactorily. However,

these turnout designs are traditionally intended for railroad use, and considering

the lower axle loads of Light Rail Transit (LRT) applications, other options were

considered.

During the evaluation of turnout options during design development, the HPO

and MnTC considered the embedded switches manufactured by Edgar Allen

Limited in the United Kingdom. Edgar Allen’s switches are currently in use by

Tampa, Florida’s Hartline system and are being installed in New Orleans’

streetcar system. They are also in use in Sheffield and Manchester, England

and Hong Kong. These “flexive tongue” designs are ideal for paved track

applications and can provide maintenance advantages. Flexive tongue turnouts

involve a solid manganese steel body with a flexible manganese tongue, acting

as the switch point, fastened to the body by two screws. The tongue is easily

removed for maintenance or replacement without removing the entire unit.

Electrical resistance heaters (calrods) can be placed along the sides or

underneath the manganese castings.

3. METALLURGY

Edgar Allen’s flexive tongue turnouts were introduced in 1992 for the Sheffield

Supertram system in the UK. The Supertram system opened in 1995, and

Edgar Allen supplied turnout points and castings for six 25m and three 100m

radius turnouts. Both the manganese points and bodies are rated at a Brinell

Hardness of approximately 250 at manufacture – Edgar Allen reports that in the

five years after opening of the Supertram system the bodies and points have

work hardened to approximately 400 BHN. The castings and points are

manufactured using austenitic manganese steel.

4. TURNOUT DESIGN CONSIDERATIONS

The Hiawatha designers evaluated other aspects of the turnout design that would

ultimately impact the performance of the turnouts. These factors should be

considered for any transit project that requires turnouts in paved areas.

a. Paving

Depending upon whether vehicles will actually drive upon the turnout, careful

consideration should be taken regarding the type of paving to be used around the

rails and other turnout components. For Hiawatha, it was decided to use

portland cement concrete “islands” between the rails, with asphalt cement

concrete infill to approximately six inches from the rail head. This would give

better support to the traffic that could potentially cross the turnout and was

considered more aesthetically attractive. Based upon discussions with other

transit properties, it was decided that using portland cement concrete against the

rail would not perform well due to movement of the rail against the concrete,

potentially breaking the concrete. It could also present problems with

replacement of rail and turnout components and might not provide satisfactory

rail to rail insulating properties.

b. Electrical Isolation

The embedded track for the Hiawatha system was insulated from the ground to

protect adjacent utilities and structures from electrical corrosion by using the

rubber boot system manufactured by the Iron Horse Corporation cast in portland

cement concrete. The turnouts, however, could not use the boot system due to

the irregular size and shape of frogs, switch castings, and plates. As a result, the

embedded turnouts were constructed in insulated concrete “tubs”, which are

comprised of reinforced concrete with a rubberized isolation liner with protection

board. The rails as they enter the tubs are themselves isolated with bulkheads

perpendicular to the track. The turnout materials, including rail, plates, frogs,

switch machines, switch castings, and the concrete islands discussed above are

installed in the tub. Rail and switch components are mounted on elastomeric

grout pads that provide additional insulation and shock resistance.

c. Switch Machines

The Hiawatha LRT system, as currently designed, has six crossovers in

embedded track. Four crossovers are powered using Hanning & Kahl (H&K)

switch machines, and the other two crossovers use hand throw turnouts. Spring

switches, although common on other systems, were not utilized for operational

reasons.

The power switches are powered by H&K Model HW(E)61V. The switch

machines are located between the switch casting and each switch machine box

is drained with non-conducting PVC pipe to the storm drainage system. Power

and signal conduit to the switch machine and turnouts run through the isolation

membrane and are insulated. Both bonded and “poly bar” insulated joints are

used and are identical to those in standard track applications. Hand throw switch

boxes are also located between the switch castings.

d. Switch Heaters

Since Minnesota winters are long and cold with plenty of snow, the embedded

turnouts will be heated using electrical resistance heaters (calrods). There was

considerable discussion within the design team regarding whether to install the

heaters along the side of the switch castings or underneath. There is no ideal

way to heat the switches, since the heaters will be in contact with other materials

such as asphaltic concrete or elastomeric grout. Discussions with other transit

properties revealed that both methods of heating are used. The Hiawatha

designers decided to place two heater rods in metal conduits under each casting,

above the concrete and far enough away from the elastomeric grout pads to

avoid overheating of the pads. The heater rods can be pulled out of the conduit

and replaced without removing the switch casting or conduit. By installing the

heater rods underneath, it avoids the inevitable melting of asphalt concrete along

the side of the casting, and the dual heater rods provide redundancy.

e. Drainage

The paved areas in downtown Minneapolis and suburban Bloomington are

generally flat and drainage is a challenge. In warm climates, poor drainage in

embedded turnouts can be a nuisance – in Minnesota, with long periods of sub-

freezing weather, frozen switch mechanisms could put a turnout out of service for

months. The embedded turnout tubs and embedded trackbed are provided

surface drainage and the switch machine boxes are also drained. Switch heater

rods will be placed in the switch drains to keep them clear during the winter

season.

5. CONSTRUCTION METHODS

At the time of this writing, the embedded turnouts for the Hiawatha LRT project

are under construction. The construction of the turnouts is very time consuming

compared to construction of ballasted turnouts and takes several months from

start to finish. The construction of the turnouts can be discussed in terms of the

three concrete pours, as follows:

a. First Pour – The Isolation Tub

The reinforced concrete isolation tub is constructed on select base material, with

conduits in place for drainage and systems (traction power and signal) facilities.

The first concrete pour is eight inches thick, with walls up to top of rail

longitudinally on the field side of the rails, and with the concrete bulkheads to top

of rail between the rails at the ends of the tubs.

After placement of the concrete, the isolation liner is placed on the bottom and

sides of the tub. The rubberized liner is heat welded and the welds electrically

tested. The liner is installed with one-eighth inch thick protection board on both

sides of the liner.

The following exhibits show the details of the turnout design. The three stages of

construction, labeled “First Pour,” “Second Pour,” and “Final Pour” are included.

The concrete pour for that phase is cross-hatched in the cross sections.

Hiawatha LRT Design-BuildPlan ViewIsolation Tub for Crossover

BITUMINOUSPAVEMENT INFILL

TRANSVERSEC OF CROSSINGL

B

B

CONCRETE(TYP.)

C TRACKL

1’ MIN (TYP.)

FLOW 2’-0”MIN. 9º31’38” PITO

NO. 6 TURNOUT WITHDOUBLE TONGUE SWITCHPER SPEC. 17031 (TYP.)

17’-1” 4’ MIN.

A

A

C

C

PS

SWITCH AND COVER PLATE

SWITCH DRAIN(AT EACH SWITCH)

DETAIL “A” (TYP.)SWITCHBLOCK OUTCONCRETE (TYP.)

2’-0” (MIN.)TUB DRAIN(AT LOW END)

CONCRETE (TYP.)

FLOW

BITUMINOUSPAVEMENT INFILL

C TRACKL

EMBEDDED TRACK SLAB

38’ MAX.POURED POLYMETRICJOINT SEALFOR LESS THAN 2”GAP IN RAIL HEAD(TYP.)

12’-3

”4’-0”

MIN.

7’-6”

MAX.

Exhibit I, Plan View, Isolation Tub

Hiawatha LRT Design-Build

Isolation TubFirst Pour

#4 REBAR@ 18’ ON CENTERALONG C TRACKC TRACKL C TRACKLL

CURB AND GUTTERAND SIDEWALK

ELASTOMER GROUT (TYP.)

6” AGGREGATE BASE, CLASS 5 (02211)6” NON-FROST SUSCEPTIBLE SOIL

ISOLATION TUB

1

3

1

6

PROTECTIONBOARD

ISOLATIONMEMBRANE2

2

4” x 4”CHAMPHER

BITUMINOUSPAVEMENT(TYP.)

3-0”6”

MIN.1% 1%

4’ VARIES 4’3-0”

6”MIN.

1%

4’-8 ½” GAUGE 4’-8 ½” GAUGELEVEL(TYP.)3” CLEAR

(TYP.)

BITUMINOUSCONCRETEIN FILL (TYP.)#4 @ 18”

(TYP.)2 1/2” CLEAR(TYP.)

PCC

CONST.JOINT(TYP.) INSET

(TYP.)

Exhibit II, Isolation Tub, First Pour


TUB EMBEDDED TRACK SLABSection D-DEnd of Isolation Slab

TOP OF RAIL

6” 2”

ELASTOMER GROUT SEALFOR MEMBRANE EDGE

1” EXPANSION JOINTMATERIAL AND SEALER

#6 BETWEEN RAILSTUBREINFORCEMENT

6”6”

EPOXY COATED#9 DOWEL @ 12”

2’-0”1’-0”#6 TOTAL 3

9”

SPECIAL TRACKWORKPLATE WITHE.G. PAD 1

#5 BETWEENRAILS

115RE RAIL

BEND UP #6 BARHORIZ. REINF.

4” x 4”CHAMFERISOLATION SLAB

ISOLATION MEMBRANETUB (WORKING SLAB)

PROTECTIONBOARD

Exhibit III, End of Isolation Slab

b. Second Pour – Support for the Rail

A strongback system and jacks are used to support the assembled turnout in the

proper line and grade prior to the second concrete pour. Anchor bolts and

inserts for the turnout plates are placed in the plates in position for embedment in

the concrete. Plastic caps are placed over each of the e-clips to prevent fouling

during the concrete pour and for protection afterward when asphalt is placed

against the rail.


Isolation TubSecond Pour





ISOLATION TUB

1

3

1

6

PROTECTIONBOARD

ISOLATIONMEMBRANE2

2

4” x 4”CHAMPHER


3-0”6”

MIN.1% 1%

4’ VARIES 4’3-0”

1%


(TYP.)



PCC

CONST.JOINT(TYP.)

6”MIN.

INSET(TYP.)

Exhibit IV, Isolation Tub, Second Pour

The concrete is placed within the tub to a level one inch below the turnout plates.

The one inch vertical gap will be filled with elastomeric grout, which provides

electrical isolation and support for the plates, after the second pour concrete is

cured. The reinforcement in the second pour is of course separate of the

reinforcement in the first pour due to the design of the isolation liner, but hoops of

rebar are included in second pour to secure the “islands” above, which are part of

the third pour.

c. Third Pour – Concrete Islands

While elastomeric grout is being poured under the turnout plates, systems work

can be done, including bond wires to rails for power feeds and signals, setting

and adjusting the switch machines, and installing and testing the insulated joints.

The third and last concrete pour brings the concrete between the rails and on the

field side of the rail against the isolation liner to top of rail.


Isolation TubFinal Pour





ISOLATION TUB

1

3

1

6

PROTECTIONBOARD

ISOLATIONMEMBRANE2

2

4” x 4”CHAMPHER


3-0”6”

MIN.1% 1%

4’ VARIES 4’3-0”

1%


(TYP.)



PCC

CONST.JOINT(TYP.)

6”MIN.

INSET(TYP.)

Exhibit VI, Isolation Tub, Final Pour

The approximately six inch gap between the ball of the rail and the island

concrete will be filled in later to top of rail, along with a flangeway, with an asphalt

concrete infill to finish the work.

6. SUMMARY

The embedded turnouts for the Hiawatha project will be operated in a harsh

weather environment and will be critical to the successful operation of the

system. The Edgar Allen turnouts, while fairly new to North American transit

systems, have been proven to be reliable in Europe and its design should

perform well in street conditions in Minneapolis and Bloomington.

Documents

Embedded Turnouts for the Hiawatha LRT Project · and Hong Kong. These “flexive tongue” designs are ideal for paved track applications and can provide maintenance advantages