Wyalusing is a good spot to watch BNSF. It doesnt have cabins, but has hiking trails by the rails and is on the Wisconsin and Mississippi rivers, so the boating and fishing is good. Wisconsin has many state parks near (20ish miles) rail lines, but most lines dont see that much rail traffic. In south central WI, the WSOR is the main road. Janesville has quite a bit of traffic, Madison sees a train or 2 a day plus locals. Many state parks nearby. I'm not familiar w/ SE Wisconsin. A good place would be in the Wisconsin Dells or Lacrosse area. Devils Lake state park is near a WSOR line that sees about 2 trains a day in the summer. It is also near Wisconsin Dells (big resort area) and the mid-contenent railway museum which has tourist trains several times daily. It is also near the 400 state trail and the elroy-sparta state trail (w/ 3 tunnels you can bike through). Wildcat Mtn state park is another park near the above 2 trails. Devils lake state park fills fast (weekends are already booked solid), but several other state and county parks are nearby. In the lacrosse area, there are other state parks and sees quite a few trains. There is a 4 or 5 way crossing called grand crossing that is easilly accessible (legally) but without amenities for viewing. The lacrosse river state trail parellels an active rail line running east from lacrosse. Last year, In about 2 hours, I saw 6-8 trains from the trail. The Stevens Point area and Fond du Lac areas also see plenty of rail service and have state parks nearby. I dont think very many of the state parks have cabins, but a quick websearch should identify private campgrounds that might.