Okay, we are talking about the toilet on board the Metrolink trains backing up and/ or not being serviced properly. Unusual since I have never seen that happen. Sounds like a passenger was quite ill that day. The commode on board is a chemical type with a holding tank. This is probably dumped once a day, and fresh chemicals added etc. etc. Okay so someone was asleep at the switch and that car did not get serviced, or you had the misfortune to get on board shortly after our ill passenger did his or her thing. Hopefully it will not reoccur.
By the way, as with airlines, Metrolink trains have odd numbers going west and even numbers going east. So if you were leaving Pomona heading for LAUS, your Metrolink train had an odd number. To quote an old joke from someone that didn't like Californians, Oddballs go West.