All about the pantographs, baby
The pantograph, frequently abbreviated to “pan”, is the spring-loaded arm that’s part of how the train draws power from the overhead wires, aka catenary.
By design, the pantographs press upward on the catenary. Where there isn’t a lot of clearance between the catenary and the train, the pantograph folds nearly flat on itself, as seen here at Sunset Transit Center.
The bow collector (presumably named that because it’s shaped like a bow) at the top of the pantograph arm is topped by a carbon shoe, which is the part that directly contacts the wire (which, not coincidentally, is called the contact wire). The carbon shoe is gradually worn down by the overhead wire and eventually needs to be replaced. To wear it down evenly, the overhead wires zigzag back and forth instead of going in a straight line which would only wear down one part of the carbon shoe and potentially break the pantograph.
This is not a TriMet video, though I’d love to set up a camera on top of a MAX train to get something similar – it shows both how the spring-loaded pan rises and falls depending on how much distance there is to the wire above as well as the back-and-forth zigzagging of the wires so that the carbon shoe wears evenly.
The Overhead Wire, aka Catenary
The overhead wire ranges in voltage from 675-925 volts, averaging around 750 volts, direct current. In other words..
DON’T TOUCH IT.
I used to have a link here to a Philadelphia news article where a 15 year old climbed a cat pole to touch the overhead wire but the link expired. He survived but was severely burned. A simple Googling shows a lot of fatalities that happened when people climbed on top of trains or up cat poles to touch the wires or the pantograph. I shouldn’t have to say “don’t touch something high-voltage” because it should be obvious that that’s a really bad idea, but I’m all about spreading the safety message even when it means stating the obvious.
Low-speed and high-speed areas
In low-speed areas, such as downtown Portland or in the yards, a single-wire trolley system is used. Throughout the downtown alignment, you’ll see how the contact wire is a single wire strung through other supporting wires.
In high-speed areas, the overhead wires look like this. The upper wire is called the messenger wire, which supports the lower contact wire.
On some of the catenary poles, you’ll see tension weights hanging. I’ve already gone over how the weights work in another post – but to summarize, they rise and fall as the temperature changes to keep tension in the overhead wire.
Throughout the alignment are section isolators (also known as section insulators) in the overhead wire. These unpowered breaks in the line allow for power to be turned off in one area without needing to shut down the entire system. To prevent arcing and other problems, a MAX operator going under an isolator won’t draw power until both pantographs are clear of the isolator.
Those pictures are examples of arcing downtown where the Yellow-Green alignment crosses the Blue & Red alignment. If you’re on a train going through where the lines cross over and all the lights in the train go out briefly, this is why. Arcing is not a desirable phenomenon, and excessive arcing can sometimes be indicative of a very serious problem.
As a visual reminder of where the section isolators are in high-speed areas where a train is likely to be in a propulsion mode greater than those used in low speed areas, there are C and P signs along the alignment associated with the isolators. When the front of the train reaches the C sign which will be located prior to the isolator, the operator must coast, and therefore not draw power from the catenary until reaching the P sign, at which point the operator can resume a propulsion mode drawing power because both pantographs have passed beyond the isolator.
In most sections of the alignment, unless you know the isolator is there, you most likely won’t be able to feel the train going into coast and back, but offhand I can think of two sections where it’s fairly obvious to passengers – westbound out of Willow Creek (where this picture was taken) and eastbound out of Sunset. At Willow Creek, the isolator is so close to the platform that the train will not pick up much speed before the operator must coast, so westbound departures from Willow Creek often feel very slow.
At Sunset, the isolator is on an upward hill – note the C sign at the base of the hill – so gravity is working against the train coasting uphill and you can feel a slight jerk and drop in speed as the train goes into coast. If the operator doesn’t pick up enough speed leaving Sunset before coasting up the hill, the ascent will feel unusually slow.
And…. I think that about covers the basics. Really the biggest (only?) takeaway message from this for TriMet passengers is don’t ever touch the overhead wires since knowing the rest of this stuff isn’t a prerequisite for riding the trains but staying away from the catenary wires is just a good idea! But as always, I think the intricacies of the system and all of the things that go into making it work are fascinating, even if most people don’t care/don’t need to care about them.