As somebody that lives in Boise, ID, I can say that the old Boise and Nampa Depots are well worth a visit. There isn't much in the way of preserved stuff, but if you decide to get off the beaten interstate, you can follow the UP Nampa Subdivision all the way from Nampa to Minidoka, ID. There are several relics worth seeing including an old water tower at Orchard, ID south and east of Boise. At Reverse on old US Highway 30, there are the only sets of searchlights left operating on this particular subdivision. Occasionally, helpers get sent out to rescue trains coming up the hill. If you continue to follow old US30, it will take you all the way to Glenns Ferry, where they are loading the sugar beet trains, and there is a old coaling tower for when the steam trains operated the line. You can follow US30 to a place known as King Hill, and there is a pull off up from where the tracks cross the bridge and gives a splendid view looking down at trains as they thunder up the 1.5% grade to Bliss. US30 meets back up with I84 a few miles from this location. You can get off at the Bliss exit, navigate in to the little tiny town, and there is another water tower from the steam days. It's a great trainspotting location as well, as crews are always friendly and giving big waves and extra horn as they come through the town. Westbound trains will always being dynamic braking unless pulling out of the siding, and Eastbound trains will be going full power. From there, you can follow US Highway 26 out to a town known as Gooding, where there are some decent railfan spots. If you follow 26 out of Gooding you'll come to a town known as Shoshone, ID. Railroading runs in the blood of this town as there used to be a yard there. There is an old depot right in downtown, and a platform just for train watchers to gather. There are 3 crossing spaced close together, so if you love horn, you're in the right place. Awesome crews are gonna give you a big show. Shoshone snack bar is a fantastic place to eat as well, and good ice-cream cones. From here, you can get on Idaho Highway 24 which you can ride all the way to Minidoka, ID. Along the line there are leftover lineside telephones. You need to see those while they still exists, it won't be long before they get removed. At Kimmamma, ID (However you spell it) there is a water tower and next to a grain elevator. Plenty of spots to pull off and watch trains, as the highway runs right along the tracks, and of course numerous side roads. At Minidoka, there is an interchange between UP and WAMX's Eastern Idaho Railroad. The interchange is easily accessible by car and makes a splendid location to watch trains because of the old water tower that sits track-side and can easily be captured in any shot you take. From here, the highway goes south towards Rupert and Burley, but if you're interested in risking your tires to make an off road run on roads lined with obsidian rock shards, you can drive at to Wapi, ID, which is an old Lava field from a Volcano in Craters of the Moon National Monument. There is an old water tower and a couple of abandoned buildings here, and scenically is a great spot. However, you're isolated from any civilization, and cell coverage is not always existent, I would prepare adequately before going out there.
Rupert is the heart of the EIRR operations. Numerous industries can be found along the line, and the area as far as I know is railfan friendly. Old American locomotives are all you're going to find operating here. The highway meets back up with I84 at Burley, ID, which is about 15 miles from the I86-84 interchange. Here 84 goes south into Utah, and 86 continues to Pocatello, ID. If you stay on 86, you can stop by American Falls, ID where the trains go over the American Falls Resivor on the Snake River. At Pocatello, there is a major yard, with a locomotive light repair center and refueling points. From there, it's on to I15 south into Utah, which continues along the line to other spectacular spots, I've not yet been to myself.
