From Wikipedia:

Supertrain is an American adventure-drama television series that ran on NBC from February 7 to May 5, 1979. Nine episodes were made, including a 2-hour pilot episode.

The series took place on the Supertrain, a nuclear-powered bullet train that was equipped with amenities more appropriate to a cruise ship. It had luxuries such as swimming pools, shopping centers, a gym, library, medical center, and a discotheque.[1] It was so big it had to run on very broad gauge track. Though it had a rated top speed of 250 mph (402.34 km/h), and cruised at 190 mph (305.78 km/h), the train took 36 hours to go from New York City to Los Angeles; this would put the train's average speed at around 80 mph (128.75 km/h), slower than that of Amtrak's Acela Express and well below the speeds of bullet trains in Europe and Asia. (Some episodes state, however, that the train also stops in Chicago, Denver, a fictitious town in Texas and presumably other cities, which would extend the length of the run and thus would require faster speeds.) Much like its contemporary The Love Boat, the plots concerned the passengers' social lives, usually with multiple intertwining storylines. Supertrain was described in a 1979 Variety review, "It's a 'Love Boat' on wheels which has yet to get on track."[2] Most of the cast of a given episode were guest stars. The production was elaborate, with huge sets and a high-tech model train for outside shots.
https://www.youtube.com/watch?v=2Av1P8cFVw8
why Rifftrax should riff on it:
1. a pool on a train!
2. a disco on a train!
3. a nuclear-powered train!
and bonus...
it's 1979! It needs to be riffed!