F40 wrote:1. What does the R after the car # mean?
It was rebuilt to interoperate with the PA-4's. Once the fleet was rebuilt, the R's were allowed to fade. One of the changes in the rebuild was a modification to control the far front door of a lead PA-4 and the far rear door of the trailing PA-4, so that those doors would not open when running longer (than 8-car) trains in short stations. AFAIK, this was never put into use.
2. In addition, I spotted only a few PA4's that have the side, digital sign. Which ones got them?
I don't know the car numbers, but unlike the MTA, PATH is quite willing to run their experimental, modified cars as part of the regular fleet. There are also cars with LED end signs, different flooring, etc.
3. How do you change the front and back rollsigns, and how many stations are featured on the rollsign?
Depends on the car model. Either via conductor's key to the left of the end door (while looking up into a flap in the car ceiling) or from the operating cab. A couple modified cars with LED end signs have a conductor's key-operated selector switch to the left of the end door. I don't have the whole sign list handy.
4. On Sept. 8, '01 at the WTC station, I spotted an array of PATH station names on the overhead ceiling area, and saw 'local', 'express', and several terminuses. Did PATH ever have express service on this line, or any other line?
The signage was to allow maximum flexibility. By the time of the attacks it was pretty dingy and mostly broken due to lack of maintenance. There was direct express service from NWK to WTC at some times.