Not an expert, but I believe 15 foot centers is the old standard which generally requires that the adjoining track is put out of service during maintenance work. 25-foot centers are used on some new tracks in the Powder River Basin for the cited reason.

However, even with 25-foot centers, there may still be times when the adjoining track needs to be taken out of service, such as when a large crane with a long boom and extended counterweight is working, or when you have an undercutter throwing out large debris clouds. But, 25-foot centers let MoW crew members get in the clear between tracks, even when there is moving equipment on both tracks.

I assume that in difficult mountain terrain, the operational flexibility of 25-foot centers is offset by the much higher cost of spacing tracks that far apart.

-- Ernest