If this is your regular train/car, please let the conductor in your car know that the ipad mini left on the train is yours. The conductor may be holding it for you. Whether you ride this car regularly or not, you should contact lost and found at NJT, describe the item, and let them know where/when it was left. Good luck!
If a conductor found it, it is likely to be in a building on the grounds of the Dover train station, near the yard where the trains are stored. I tracked an iPhone there once that was left on the train, using the "Find my iPhone" feature. I think they have some bins where they put the stuff they find before eventually sending it to the lost and found in Penn Station. I was lucky I found it while it still had battery life, because after that, tracking it would have been impossible.
If anyone found my iPad mini on the 6:18pm train, please let me know. Thanks.