You'd have to be a determined DIY'er and some soldering would be involved, but a few years ago I posted (I think it was here on OGR) how to do this for about $5 in parts/modules. Basically you use $1 eBay RF/wireless modules. When the front powered car is in reverse, the power to the reverse lamp (MTH PS2 electronics) powers a transmit module. The car on the other end has a $1 receive module that detects this radio signal and drives its directional LED lights.
The limitation is/was this would only work on one subway set at a time since otherwise all end cars in all consists would detect the reverse radio signal. There are workarounds to this but that's why I said you have to be a determined DIY'er. If you're seriously interested, I can dig around for the info.
