Not much traffic on the line to Kolari, basically the night train and one freight per day. Kolari itself is a small town (Lapland small town = village), and the station itself is basically a rail head for the surrounding ski resorts. Rovaniemi and Kemijärvi are much larger towns, with Rovaniemi being basically *the* city in Lapland - plenty to do there, including some very good museums, restaurants and the near-by Santa Park etc.
Tomorrow, for example, Rovaniemi seems 6 departures southbound - if you're looking to visit Lapland then Rovaniemi is the best place to go.
If you want to see what is running and in a lot of cases what motive power, check out juliadata.fi which collates all the public data from Finntrafic. You'll have to figure out the Finnish, but "kartta" is map, and juliadata has a very nice live map. Works on mobile too, YMMV with Google Translate. It does show all passenger, commuter, infrastructure/work and freight trains.
Turku is worth a visit for the town itself and the new main station. Kouvola...depressing industrial city with big rail yard. If you want Sr1s then take the commuter train from there down to Kotka - maybe jump off at one of the intermediate stations, it is a very busy freight line and in the progress of being modernised ECTS - I know people who have worked on it and the capacity and capabilities of the signalling system are impressive. For example, everything passing through Inkeroinen tomorow:
https://juliadata.fi/timetables?s=IKR&d=7.1.2026 T=tavara (freight), HL=henkilölikenne (commuter), VET=veturi (locomotive, light engine). High probability of Sr1s.
Your other option is heading up to Tampere (much nicer than Kouvola) - there's a big rail yard there south of the city centre with quite a few Sr1s hanging around last Sunday that I saw. Toijala and Hamenlinna are major towns along the line with both commuter and InterCity services, here's an example of the freight traffic through Toijala for tomorrow:
https://juliadata.fi/timetables?s=TL&d=7.1.2026