Section 4: Track DesignAnchor: #i1005644
Most railroad companies prefer that any required track work to support a TxDOT construction project be done by a TxDOT contractor. Railroad companies usually assist by:
- cutting rails
- welding or bolting back together new track sections
- installing switches
- relocating signal equipment.
However, this is not common with all railroad companies. Project designers should clarify who will provide materials, remove existing track sections, dispose of materials, and install new materials prior to developing plans and specifications. If this is not done properly up front, TxDOT may be at risk of a change order during construction.
The goal of effective track design is to transform the intense load of the railroad equipment (locomotives, cars, etc.) from the wheel-rail contact point to a moderate, distributed pressure that the earth underneath can support in all weather conditions without settling. This is accomplished using the components in the following subsections (“top down”).Anchor: #i1005684
Rails are classified in weight per linear yard of a single rail (i.e. 132# rail).
The size of rail to be used in a particular project is typically the choice of the operating railroad company. That decision will usually be driven by either using the rail section currently used by the railroad company for their new construction, or by matching the size of rail that exists on that particular segment of track. Heavier weights are typically used on mainline tracks, while smaller sizes may be used on spur and siding tracks.
A railroad company may also specify the use of a particular material, such as head hardened rail.Anchor: #i1005704
In order to facilitate accelerated construction, railroad companies often deliver preassembled track panels to a job site. These panels are typically 80 feet in length and include:
- tie plates
Track panels are particularly useful when replanking an existing at-grade crossing. After the existing rails are cut on both ends, an existing section of track is removed down to the subgrade. After the subgrade, subballast, and ballast are laid down, the track panels are installed, the track panel is bolted on both ends, and a tamping machine installs ballast. The temporary joint bars are then removed, and the rails are welded together. Any crossing surface panels are bolted into the ties.Anchor: #i1005734
Securing the rails to the ties is usually done by the use of:
- tie spikes and tie plates for timber ties
- tie clips and rail seat pads for concrete ties.
Tie plates and clips also assist in distributing the load of a train over the tie.Anchor: #i1005759
Ties are typically timber or concrete supporting members to which rail is fastened. They provide distributive support for the rail and assist in maintaining track alignment and separation between rails.
Timber ties are manufactured from hardwoods such as oak or Douglas fir and pressure treated with a creosote/tar solution to prevent decay. Timber tie size is usually 7 by 9 inches by 8 foot 6 inches. Timber tie spacing is usually 18 inches or 19.5 inches on center.
Concrete ties are prestressed with rebar, resist decay, and generally have a longer useful life than timber ties. Concrete tie size is usually 11 inches by 9 inches by 8 foot 6 inches. Concrete tie spacing is usually 20 inches or 24 inches on center.
Switch ties may be timber or concrete ties of varying lengths (generally 9 feet to 20 feet) that are used to support the track structure at the location of a turnout where a track diverges into two or more tracks by means of the turnout and switch mechanism.
In recent years there has been increasing interest in the development and use of ties made of composite materials or primarily polymers mixed with timber or concrete. Composite ties are not in general usage by any of the major railroad companies, though there are some installed at various locations around the country as “test” projects.Anchor: #i1005789
Ballast restrains the movement of crossties to prevent lateral movement of the track structure, provides distributive support to the crossties, and provides drainage for the track structure above. Desired ballast depth is within 2 inches of top of the ties and 9 to 12 inches below the bottom of the ties. The width of the ballast section is usually 6 to 12 inches from the ends of the tie with a 2-to-1 slope downward from that point to ground level.Anchor: #i1005799
Subballast consists of smaller particles that provide for additional support of the track structure and a foundation course to further aid with drainage. As track structure ages and upper ballast deteriorates, those smaller particles “migrate” downward, effectively deepening the lower ballast layer. Maintenance and rehabilitation projects add newer upper ballast, causing the entire track structure to gradually rise vertically over time.Anchor: #i1005809
On new construction, core samples are usually taken to determine what type and what depth of constructed subgrade (compacted aggregate) should be used to support the track structure. On existing older rail lines, the subgrade is typically the native soils which were present when the railroad was built.Anchor: #i1005819
Track Design Standards
AREMA has standard track components and design plans that are often referenced for track design. The Class 1 railroad companies (BNSF, CN, CP, CSX, KCS, NS, UPRR) also have their own specific engineering and design standards for some components and design that may vary from AREMA standards and supersede AREMA standards. Some of these are also “common standards” that are used by more than one railroad company, such as BNSF/UPRR common standards.
TxDOT has permission from UPRR and BNSF to use their standards and common standard sheets in PS&E packages. The designer should confirm from the railroad company which standard to use prior to developing any track design plans. Track design should only be performed by individuals with prior track design experience who are familiar with both AREMA and railroad company standards.Anchor: #i1005834
A turnout is an assemblage of various components that diverts trains from one track to another. The length of the turnout is determined by the angle of the turnout casting (referred to as a “frog”). The shorter the turnout, the sharper the angle of divergence will be, which restricts train operating speeds through the turnout. The initial point of divergence where the two tracks effectively meet is referred to as the “switch” and the moveable rails at that location are referred to as a “switch point.”Anchor: #i1005844
Grades and Horizontal Curves
Railroad optimum design for grades is 0.5% or less, though up to 1.5% is acceptable in certain circumstances. Steeper grades result in continual reduction in train speed and can actually cause a train to stall if the grade is too steep, or require the use of additional locomotives, which is a major operating cost.
Horizontal curvature is not nearly as critical, though it can affect train speed and handling. The degree of curvature and desired train speed will impact the amount of superelevation used. Consecutive curves also require the design of a tangent length of track between curves to prevent the train from rocking, which can result in derailments.