Stay hydrated and check before you go
With the temperature expected to soar tomorrow, TriMet reminds riders to prepare for the heat and stay hydrated. High temperatures means trains will slow down in some sections and could lead to some delays.
Trains run slower in higher temperatures
When temperatures pass 90 degrees, TriMet reduces train speeds by 10 mph for all areas with a speed limit of 35 mph and above. If temperatures reach 100 or higher, we reduce all train speeds to no faster than 35 mph.
Keeping riders safe/Preventing system damage
Our goal is to keep riders safe. When temperatures start to rise TriMet trains slow down as a safety precaution. High temperatures can cause the overhead power wires (which are made of copper) and the rails (which are made of steel) to expand.
The overhead power wires may expand in the heat. Because copper expands more than steel, and because we can’t allow the overhead wires to sag, we have a system of pulleys with counterweights that tug on the wires to keep them tight. Sometimes, it gets so hot that the counterweights touch the ground and the wire starts to sag anyway.
Additionally, TriMet has larger sections of ballasted trackway (areas where our rail ties are set in rock and not embedded in concrete).
When many miles of rail are subjected to intense heat, the rail becomes incredibly hot. The stone track-bed and the consistent lack of shade add to the high temperatures. Since the rail is firmly anchored into the railroad ties, it has little room to move.
Heat causes expansion and the superheated rail can increase in length measuring in several inches over a great distance. Since there cannot be any gaps in a rail to allow for this expansion, pressure builds up in the rail as it tries to expand lengthwise but can’t. With no room to expand, the rail can bow, known as a “sun kink” or “heat kink”.
Our operators have to watch for both sagging power wires and “sun kinked” rails when it’s really hot out. To be safe, they slow down to make sure nothing goes wrong. As it gets hotter, they have to slow down more.
Find more information at: http://howweroll.trimet.org/2012/08/22/ask-trimet-why-do-max-trains-have-to-slow-down-in-hot-weather/