Just wondering, but what are the minumum standards / requirements a station must have before Amtrak will agree to stop there?
This is a bit complicated. Amtrak has different standards depending on expected ridership.
First of all, for a new (as in, not currently open) station, the FRA's current ADA regulations would have to be satisfied. These are all about the nature of the platform; no station building is required.
Second, the host railroad's requirements (in this case BNSF's) have to be met. These are somewhat arbitrary -- they vary depending on the whims of current BNSF management.
Finally, there are Amtrak's actual standards. These are a bit complicated. Amtrak has different "station standards" based on expected ridership in order to provide some level of service quality. These are certainly the most "open to negotiation" of the three hurdles, and therefore the least important.
In terms of facilities provided, there seems to be quite a disparity in standards between different stations that cannot be explained away by their significance alone. I guess the expalanations must be more historical.
Entirely historical. Any station which has been in continuous operation is grandfathered to a certain extent, causing results like Sanderson, TX, where the station was demolished before Amtrak. And of course even in cases where Amtrak would like to enhance station facilities, it often can't get the money or legal agreements together to do so.
Also, if the train must stop on the main line, BNSF would have to agree to that and might have reason to object. This might be a reason to build some sort of siding so freight trains can overtake, but again this hasn't been done everywhere so it seems to be done on a case by case basis.
Basically, it's a matter of whether the host railroad demands it at that location. For existing stations, the host railroad is stuck dealing with whatever currently exists, but when major renovations are planned or new stations are built, sometimes the host railroad demands trackwork changes. CSX demanded a *pair* of sidings at Rochester, NY, for example. There's really nothing to be done other than agreeing to such requests. The only way to know whether BNSF will demand a passenger siding at a given location is to ask them.
So from Amtrak's point of view, what sort of station does there have to be before they will stop. Is a bit of hard standing and a sort of shelter and a station sign good enough? Or does soembody have to spend millions before Amtrak will even consider it a station?
From Amtrak's point of view, anything which passes muster with the FRA, the DOJ, and the host railroad is good enough. Though they'd prefer to have something which matches up with their service standards based on ridership. For a low-ridership stop this means a flat platform of a legal height with a yellow tactile "watch out" strip at the edge. Amtrak only asks for any sort of shelter or building at fairly busy stops, and even then Amtrak will still serve the stop if it doesn't have one.