"and one could probably make the argument (legally) that the usage of a tripod by a by-stander (railfan) on a platform is not in violation of Amtrak's policies."
No, you couldn't, because as you note in the preceding clause, being a rail fan on the platform without a ticket is a violation of Amtrak's policies.
(well I suppose you could, you can make any argument, but it won't be correct)
The usage of a tripod by a non-ticketed member of the public is not against Amtrak policies. Where you decide to film, regardless of what equipment you use; is restricted. Not the equipment.
So if you were filming with a tripod on a platform; the policy gives you right to use the tripod, it doesn't allow you to film where you are. If they kicked you out for "using a tripod" then they would be going against their policy.