Rail terms

What is…?

Glossary / dictionary of rail terms, some specific to MAX, others pertaining to railroad systems in general.

MAX – short for Metropolitan Area Express, this is the light rail system in Portland, Oregon operated by Tri-Met. Presently consists of four lines designated by color (Blue, Red, Yellow, Green) on 52.4 miles of track. Fleet consists of 127 train cars manufactured by Bombardier and Siemens. Utilizes an overhead catenary system for power.

TriMet (previously written as Tri-Met) – short for Tri-County Metropolitan Transit District of Oregon. This is the public transit agency of the Portland metro area, and service consists of bus, light rail, and commuter rail (affiliated service includes Portland streetcar)

Light rail - indicates high capacity loads at lower speeds than heavy rail lines. Does not mean the rail cars themselves are light (they are not – one MAX train car weighs approximately 50 tons)


APACU – Automatic Passenger Announcement Control Unit – controls the external signs and audio and readerboard announcements on the trains

ATS – Automatic Train Stop – a device found between the rails along the alignment that will be active if an associated ABS or ABS/pre-empt combination signal is red or if a train is going above the posted speed limit. Passing over this when active brings the train to a stop by automatically applying the brakes.

CBD – The Central Business District, also known as downtown Portland

Disc Brake – (aka Friction Brake) – A mechanical brake located on each wheel axle of the train

Dwarf Signal - Protects mainline power switches while running reverse in ABS territory. Can only display a red aspect, not seen or used while running normal traffic.

Dynamic Brake – The primary method of braking on MAX trains where the motors act as generators converting the forward motion of the train into electricity, thus slowing the train.

Facing Move - When the points of a switch face an approaching train

Manual Block – A method of governing train movements on one track when the adjacent track cannot be used. The single track is used for travel in both directions as directed by Control.

Paddle – A schedule used by the operator of a train

Restricted Speed – 20mph or the posted speed limit, whichever is less, but always at a speed that will allow the operator to stop within 1/2 their sight distance

Reverse Traffic – Train movement against the normal direction of travel (e.g. east in the westbound track)

Special Instruction - A modification or addition to operating rules issued by Control for conditions lasting more than 24 hours.

Sweep – The first train to go through alignment that hasn’t been in service for a while (e.g. the first train in the morning or the first train through after an emergency situation) is a sweep train that will go no faster than 25mph to check the condition of the alignment & the overhead wire

Trailing Move - When the points of a switch face away from an approaching train

Train Order – A modification or addition to operating rules issued by Control for conditions lasting less than 24 hours. Can be in verbal or written form.

Train-to-Wayside Communication – The telecommunication system that integrates transponders under the trains with call loops in the track, through which operators can select routes, call signals, and throw power switches

And then definitions that make the most sense in clusters:

Signal terms all in one go-

ABS – Automatic Block Signal System, a type of signaling used on most of the MAX alignment where the movement of trains through consecutive blocks is governed by ABS signals. ABS signals can display red, yellow, green, and lunar aspects. Intersections with car traffic are protected by crossing gates.

block – the distance between two ABS signals

aspect – the lit part of a signal (e.g. red aspect, green aspect)

indication – what that aspect means (e.g. red indicates STOP)

Pre-empt – The signal system used where the trains run in mixed traffic with cars. Intersections are signalized with pre-empt signals that can display a yellow horizontal or a white vertical aspect.

ABS/Pre-empt Combination Signal - A signal that is capable of displaying both a red aspect and pre-empt aspects (yellow horizontal, white vertical, and in some cases a white diagonal). Used in pre-empt territory to show diverging routes, protect switches, or wherever a train may need to be prevented from moving forward.


Pantograph & related terms in one go-

Pantograph - A hinged device on the top of the trains that collects electrical current from the overhead wires

bow collector – bow-shaped part at the top of the pantograph

carbon shoe – carbon part on top of the bow collector that directly contacts the wire

Catenary – Something of a catch-all term for the overhead wire system. In high-speed areas, the upper wire is called the messenger wire and the lower is the contact wire. In low-speed areas, a single-wire system is used.

Section Insulator – An unpowered section of the catenary system used to electrically separate segments of the system

Arc – Undesirable phenomenon of an electrical current flowing through air

One response to “Rail terms

  1. In area’s where single contact wire is used (CBD) for example, the wire is called a “Trolley Wire”, single contact, direct fed from substation. As the Messenger wire is connected to the substation directly and it transfers the voltage to the contact wire. With no load on the wire (no train) the nominal voltage is 900Vdc, with a train can be between 750Vdc to as low as 600Vdc.. Note, the rail is the ground return for the overhead wire.

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