More concretely to the OP’s question:
As I understand it, Russia is principally using the railway as its backbone for moving materials into Ukraine, for the war effort.
I would note that it’s quite easy to form strong, transparent materials into pretty much any shape that you could ever want and trains aren’t really designed for speed bumps. You would need to do some engineering but I suspect that you could find a material, shape and placement strategy for this.
A single individual with a large backpack and some superglue could, potentially, take out Russian logistics for a couple of days, just by taking a trip over the border and getting a train to derail as near to the border as possible. Note, you need to be certain that civilian transport is not targeted. But you can send a map outlining target areas for potential compromise - where civilians are not guaranteed to be able to travel by train safely - and you’ll make it a lot more difficult to get workers to move around since they’ll all want to fly.
If you continue on, further, into Russia probably the easiest target to take out with a small group of sappers would be raised power cables.
Use of Molotov cocktails for arson, inside Russia, would make anyone purchasing bottles of liquor look suspicious - which will cause a lot of difficulty among police and the general public. According to some googling, about 90% of bridges in Russia are of wood construction, so that would probably be a prime target.
Realistically, you can cause issues as simply as stopping a car in the middle of a major street and slashing all the tires. Do that often enough and it’s going to be a major headache. There are only so many tow trucks in the nation.
In general, Russia is very large and has to transport things long distances. Even if you can confine the targeting area to specific lines on a map, those are very long lines and guarding them would eat manpower.
A few groups of 2-4 guys, roaming around through Russia, could cause quite a bit of slowdown to Russian logistics. From a strategic standpoint, there’s a lot of bang for the buck. The one downside would be that Russia can do the same back - except Ukraine is already blowing out its own bridges because their focus is defense not offense; they can get all the free replacement wiring and transformers that they could ever want from the West; and ditto on quick-deploy bridges. Russia has to make their own when stuff gets broken. Ukraine has the whole world cranking stuff out of factories for them.
I might also note that a lot of these are the sort of activities that you might even be able to convince some of the locals to join in on. A non-zero percentage of the Russian public are on Ukraine’s side and mucking up the system is fairly benign.