Quote:
Originally Posted by bnk
Isn’t Metra diesel electric pretty much electric already ?
The Diesel drives the electric motors that run those trains I didn’t notice a major acceleration problem compared to other systems.
https://en.m.wikipedia.org/wiki/Dies...c_transmission
It seems to me that the diesel electric locomotive us are basically a portable electric power station |
Yes, American diesel locomotives commonly use a diesel-electric transmission, but the "diesel" part of that equation imposes significant penalties on the overall weight of the train, the output of the generator, and thus the amount of torque the electric motor can produce.
Train performance is described by power to weight ratio and adhesion factor. The power-to-weight ratio of an EMU is roughly three times that of a diesel locomotive and passenger cars - it weighs hundreds of tons less, no need to haul heavy diesel fuel and generator, plus modern EMU designs are just generally lighter, using crumple zones like an auto instead of tons and tons of steel to make a perfectly rigid carbody. (These modern designs are technically illegal in the US because of our outdated ideas on train safety, but FRA often issues waivers to use modern designs under certain circumstances).
The adhesion factor is the other piece, and it is also much higher for an EMU, since there are many drive wheels spread throughout the train instead of just a handful up front. The adhesion factor explains why an EMU has significantly better performance than just an electric locomotive hauling unpowered coaches.
So why use locomotives at all? The advantages of EMUs are most profound when the train is either stopping frequently, or traveling at very high speed. So either for urban rail operations with closely-spaced stops, or high-speed rail above 150mph. For regional and intercity services with stops miles or tens of miles apart, where very high-speed travel isn't warranted, the locomotive-and-coaches model still makes a lot of sense. It's very easy to take apart and re-assemble trains and cheaper to maintain them. Some Metra lines with wide stop spacing (Heritage Corridor, Rock Island, NCS) probably don't warrant the extra expense of multiple-unit trains. I'm honestly not sure why South Shore continues to operate EMUs rather than just install better vent systems at Millennium Park.