Here on Long Island they still have a few lines that use the older style diesel electric trains.
Sometime around 2000 they phased out their diesel fleet.
The DE/DM30's are relatively new. The old diesels were much louder and smoke from the cab car would stain the tops of the car black. I tried to find a photo of one of these sets with the black tar on top but couldn't. People didn't like the old diesels and many had to take showers after getting off of them because they were so dirty.
The DE30's were originally intended to operate completely in electric mode when electricity was available, but the units had a problems of catching on fire so electric mode is only reserved for the tunnels. Most of the time these units don't even go through the tunnels.
The Long Island Railroad is the busiest railroad in the nation. They are so crowded, that they don't have enough room to accommodate more trains through the tunnels during rush hour.
What is needed in the city and in the suburbs is an extensive monorail system like the one that is now in use at JFK Airport.
This is a tough one because existing subway track already exists. While the existing system is old and not as efficient, some cities have problems where there are different modes of transportation and things aren't compatible with each other. Still, switching over to the monorail would be great if it could be constructed over the LIE since right of ways are nearly impossible to acquire. What is interesting about the JFK monorail is that it uses a linear accelerator motor and has less moving parts that can break.