Mid life overhaul may be a better way to go, unless the cars have suffered significant mechanical or structural problems. Even cars (in northern climates) that have rusted out from salt and snow have successfully undergone mid life overhaul and lasted another 10 years.
Frame cracks are another story, although I only hear of that problem on buses, which suffer the disadvantage of bumpy roads that goes all the way back to day one when they were called omnibuses and which disadvantage probably begat rail transit in the first place.
In today's social climate, even plastic windows must be regarded as expendable items, namely intended to be replaced every so often when they become fogged up by the scouring off of graffiti or going through a non-brushless carwash numerous times. The plastic windows can be buffed and polished but few artisans do that nowadays.
Electric propulsion equipment can last an amazingly long time, witness all of the streetcars and trolley buses that left the U.S. for "third world countries".
Suggest putting Scotch Magic or similar "permanent" tape over the seams of the envelope to prevent machinery jamming of or tampering with important mail.