A lot of what you might wish to see would depend upon your interests. With that qualification, here are some of the things that I have found interesting. The following narrative is from WEST to EAST in direction. You certainly could go east by one route and return west by a different route.
Let us consider 3 alternative routes:
#1 I-90 all the way, Spokane to Billings
#2 Highway 12 from Garrison to Helena to Townsend, then 287 back to I90 at Three Forks
#3 Continue east on 12 all the way to Roundup, then south on 87 to Billings.
I will follow each of my interest points WEST OF GARRISON with the alternative number. To fully understand, I would suggest getting a Montana highway map.
Wallace Id: Restored NP depot, downtown. Beautiful restoration and 2 stories of good RR stuff to look at.
From just about 3 miles into Montana all the way to Butte, I-90 generally follows the route of the abandoned Milwaukee mainline. The abandoned BN Wallace branch grade is also apparent and goes under the interstate just below the summit. Some MILW trestles and tunnels, along with cuts and fills, are visible from I-90. all alternatives as far as Garrison, #1 only east of there.
Alberton. Was a MILW division point in electric days. Large depot still there, senior center now.
Primrose, just NW of Missoula, is the site of a still standing MILW substation. Exit I-90 at Frenchtown MT and follow the MILW grade south of the big paper plant. You can continue on into Missoula on paved roads.
In Missoula both the NP and MILW depots are worth seeing. Both in nice condition. Next to the NP depot 4-6-0 1356 is on display. MRL has a large yard here. At the fairgrounds a Willamette logging engine (similar to a shay) is displayed along with a MILW depot moved in from Drummond.
About 30 miles east of Missoula the MILW substation at Ravena is visible across the river.
Another 30 miles brings you to the MILW substation at Gold Creek. You can drive right up to it. These 3 substations are the only survivors left on the Rocky Mountain division.
Garrison is just east of Gold Creek. Take hwy 12 if pursuing alternatives #2 or #3. We will continue with alternative #1.
Deer Lodge - take the business loop through town. The former MILW depot is still here, nicely done up as a church. At the south end of town little Joe electric E70 is on display. Other things of interest in Deer Lodge are the Grant Kohrs 1880 ranch, the Montana territorial prison, and the Towe Ford museum. For a small town, Deer Lodge has a lot to see. It was also the site of the MILW main electric shop for the Rocky Mountain division, but nothing remains of this facility. You could easily spend a whole day in Deer Lodge.
Anaconda. Just off of I-90 to the west. The former BA&P (now RARUS) roundhouse and yard are straight out of the 1920s. Fenced and inaccessible, but you can drive around and peek through the fence. They run a dinner train, but probably not in winter. [www.copperkingexpress.com
Butte. The original MILW passenger station remains, now a TV station. The small 1956 replacement is also still there, complete with ex MILW rails running by it; a spur to an industrial park south of town. RARUS has a small yard here, usually some geeps are there. RARUS 301 is former NP GP7 560 (BN 1634). NP 2-8-0 #25 is on display here. Not sure if former GN station still exists. East of Butte you can see the long out of service NP grade as it climbs the west side of Homestake pass. You can also loop south of Butte and follow the MILW grade as it climbs over Pipestone pass.
Three Forks. This was always a great place for MILW pictures. The sun always seemed to be right. There is a decent museum here with a lot of MILW stuff. Alternative #2 rejoins us here.
South of Belgrade at Gallatin Gateway, the former MILW hotel for Yellowstone park has been restored.
Bozeman. NP depot is still here. Montana State University is the home of the Ron Nixon photo collection.
Between Bozeman and Livingston I90 closely follows the MRL (ex NP) mainline. Most trains have helpers. Both ends of the tunnel at the top are accessible by road.
Livingston. Restored ex NP depot is now an art museum. Beanery next to the depot is open and worth a stop. Many Warren McGee pictures on display here. Warren still lives in Livingston. The Park County Museum has a NP caboose and lots of RR stuff in the museum. Former NP shops and yard are here. Lots of MRL diesels can be seen. Generally good viewing from public property.
From Livingston to Billings you are in the Yellowstone River valley and the MRL mainline is generally not too far away. This area is quite scenic and there are many good picture opportunities available.
Laurel is the location of a large MRL yard and a BNSF welded rail plant.
East of Garrison you closely follow the MRL mainline. The rr crosses Mullan pass whereas the highway crosses MacDonald pass. I have driven to the west portal of the rr tunnel under Mullan pass, but this could be iffy in the winter! On the east side you can take a short side trip to Austin where there is a big track loop. Have not been there by road myself. MRL has a small yard and engine terminal at Helena and helpers are used over the pass. NP 4-6-0 1384 is on display next to the depot. Helena is the Montana capital and I am sure there are good museums and other things to see here, but I do not have any specifics.
Between Helena and Three Forks the highway initially follows the MRL as far as Toston. Alternative #3 leaves at Townsend and continues east on highway #12
ALTERNATIVE #3 - highway 12 to Roundup, then highway 87 to Billings.
This route generally follows the MILW mainline much of the way, BUT about 30 miles east of Townsend, go south on highway 89 about 8 miles, then east on state route 294.
You could also go another 5 miles on 89 to Ringling where the MILW had a small yard and I believe the depot is still there. Route 294 crosses the summit of the Belt mountains and the MILW had a substation at the summit at Lennep. A 1956 line relocation moved the mainline about 1/4 mile south of the substation. You rejoin hwy 12 at Martinsdale.
Two Dot was another substation location.
Harlowton was the east end of the MILW Rocky Mountain division electrification zone. Boxcab E34 is on display here. The depot remains and the small "extension cord switcher" is displayed here. The roundhouse also remains, converted to some non RR use. The restored Harlo hotel is right out of the 1930s and worth a visit; good dining room. Also a decent museum with a lot of MILW stuff. They may have some booklets by Bill Wilkerson. He was a MILW Engineer who ran between Miles City and Harlowton and an EXCELLENT writer. See [www.milwest.org
for more information on Bill Wilkerson.
Harlowton was also where the MILW secondary mainline to Lewistown and Great Falls originated.
East of Harlowton the highway closely follows the MILW grade. Some of the small towns still have depots, converted to homes or other uses. At one place there was a steam loco tender sitting on disconnected track and used for water storage.
Roundup was a coal mining area, but I am not sure what remains to be seen and if it is even worth the extra 30 or so miles to go there.
This listing is far from inclusive and I am sure that others could add much to it. Also, those things which I might find interesting might be of no interest to you. We are all different. Since, in your own words, you "I watch, chase, and photograph." I am thinking that you would find Mullen pass and Bozeman pass the most interesting; also the Yellowstone river valley east of Livingston.
Have an enjoyable trip.