There’s a lot of large assumptions that go into the idea that this wouldn’t cost Amtrak anything and wouldn’t delay trains.
Does the fitness instructor do this for free (out of the kindness of their heart), or do the passengers have to pay? Who handles payment, and how is it done? If it’s done directly from the passenger to the instructor, then that delays the start of the class by how long it takes to do all the payment. If it’s done ahead of time, that opens up a whole new can of worms (i.e. passengers paid for a workout, but due to the train running late, the class had to be cut short and now they want a refund).
Also, a fitness instructor probably has a fairly tight schedule (going from one class to another), and train delays would throw that all out of whack. Unless they were paid a heavy premium to always be available (which is going to cost *somebody* a lot of money to cover that guarantee), they probably wouldn’t be that interested.
Generally, the one-night LD trains don’t have any stops that are anywhere near long enough for something like this to make sense. Even the so-called “smoke” stops (which are really just stops with longer dwell for higher ridership turnover and/or enroute recovery time) aren’t all that long for something like this. The longer stops on two-night trains are for inspection, servicing, locomotive fueling, etc., and in those cases you have all sorts of necessary activity on the platform with trucks, carts, mechanical personnel, etc., taking away room that might be needed.
Then there’s the question of liability. Who is responsible if a passenger gets injured during one of these fitness classes? Who has to ensure that the space is properly maintained to minimize injury. If we’re just talking out on the platform, those aren’t always the best-maintained surfaces, and ballast blown onto the platform, equipment laying around, etc. could pose a hazard.
Given the typical ridership I’ve seen on long-distance trains, I’d be surprised if you ever get more than 1 or 2% of passengers on any given train, and that depends on how much it’s going to cost. When I go to fitness classes in a gym (pre-COVID times), the walk-up price was around $25 for a one-hour class, and that was in a fixed location (instructor didn’t have to go anywhere to switch from one class to the next) with up to 30 attendees. Charge enough to make it worthwhile for the instructor, and you price out a lot of people that might want to do it. Make it more affordable and, given all of the above, it’s not worth the instructor’s time.