In military terms attacking rail or roads to slow supply is called interdiction. It's easier to break rail lines than roads, but the Russians have dedicated forces that keep the rail lines moving. Russia recognized a long time ago that their military was more dependent on rail than any other nation so they developed these units (during the Soviet era) to ensure the rail lines stay in good repair. They are some of the best quality and low corruption units in the Russian military.
It would be most effective to take out bridges along the route. These rail support units can replace them, but it takes any unit more time to replace a bridge than it does to just replace broken rail.
A rail line close to the front lines but out of range of artillery could be interdicted with drones. Russian rail lines are very dependent on junction boxes every few km that have electronics in them. These are used to operate switches to redirect trains as well as monitor the tracks and send information back to a central office.
In the early days of trains they often would run into one another on crowded tracks. One train ends up running a little late and that puts all other trains in the area that might be using that piece of track that day in danger. Having someone know where all the trains are at one time and diverting trains that are getting close to another train is an important function. In the US the telegraph was first run parallel to all the rail lines for a reason. Their primary purpose was to communicate where the trains were to other stations on a rail line. When they weren't doing that they were free to communicate other messages which became a profit center for the companies running the telegraphs.
Modern rail systems have automated all these functions, but humans in a command center still monitor rail traffic and are tasked with diverting trains when two get too close to one another. Sidings exist to park slower moving trains to allow higher priority or faster trains to pass through. They also exist to allow trains moving in the opposite direction to pass one another.
The Russians rely on these junction boxes for monitoring the trains. In other countries there are monitoring devices too, but they are less obvious. I think a lot of countries have switched to GPS and the engines telling HQ directly. Take out these junction boxes and the Russian rail system will become much slower and have difficulty operating. Flying kamikaze drones into the junction boxes would make life difficult for the operators of the new rail line. The kamikaze drones could also be flown into sections of track. Subtle damage to the track won't be detected until a train hits that section and derails. The Ukrainians can also send special forces in to remove spikes from the rails which would also cause subtle damage that would be hard to detect until a train comes along.
It would be difficult to keep the rail line shut down, but Ukraine could tie down a significant number of rail troops maintaining the track and force the Russians to run out of spare electronics for the rail junction boxes. If a large number of rail support troops are bogged down trying to keep that line open, longer range drone attacks can be used to disrupt rail traffic in Russia.
I'm sure this has occurred to the Ukrainians, but they are holding back until the Azov route is done. Don't tell the enemy what you are capable of doing until you're ready.
As I understand it, the rule that allows Marjorie Taylor Greene to make the motion also gives her the power to call for a vote on his ouster at any time. There are only a hand full of Republicans who would vote to remove him. MTG is holding off the vote to see if he does what she wants in the future. She made the motion as a warning to him. If a vote is actually held on Ukraine aid, then the Democrats are telling Johnson they will vote against removing him. If the motion to remove him comes to the floor before any vote on Ukraine aid, they will vote with MTG and her allies to remove him.
It's a valid threat that tells Johnson that he is safe from the most radical members of his caucus if he does some minimal things the Democrats want.
