See I actually like this guy for coming out with a decent answer.
To clarify, the goal would be to try to cause enough panic and get enough soldiers into the political heartland to force European countries to reject American support or surrender the American bases, and sign over control of the armed forces - not to militarily occupy or neutralise their armed forces. So far, European support is responsible for tens of thousands of excess Russian deaths, and I don't think they're going to just say "Ah, well, we'll get em next time guys"
Ukraine has, to my knowledge, also replaced several lines with European lines - so they could launch from Ukraine itself, specifically Lviv.
The train guys would be an isolated unit - to my knowledge, there's no command mechanism for anyone to stop the train, and once it disembarks it would do so in a visible, public place and an area where they would have several hours of being unchallenged. There could be one, in theory, but by the time anyone figures out whose responsibility it is, the train could be in a major city.
The overall goal is to make the country surrender before anyone even has a chance to fight against it - like a gambit. It doesn't necessarily have to be a train - civilian airliners, cargo ships, just some means of circumventing the NATO "shield" and bringing the fight to a European capital from the get-go. With that in mind, the battle is no longer a "paper fight" and now, all civilian losses will be of whichever European country - as you say, Austria, or Germany, or Slovakia, whoever is unlucky enough.
But this works too. This is the other extreme. A full invasion force, using the trains. The same logic applies to it - the train may be able to achieve 120km/h up to 200km/h, and has a direct route into the major cities by design. So instead of trying to launch attacks against Russian tanks in a column, you have to now launch airstrikes against Russian tanks in the streets, in your case, Parndorf. If it was an armoured train, not just a freight train, then it could even bring SAMs and shit with it.
That's where time comes into it. Trains go very fast. Parndorf is only 20km away from the border with Hungary - which is assuming that the Hungarians would allow this.
I was going off the idea that it would be from Ukraine to Poland, since Poland is the largest target and the one which would be able to retaliate the hardest. By my maths, if the reaction time was an insane 30 minutes between the border being crossed and the train being diverted, it would be in Lublin. If it was around 90 minutes, it would already be in Warsaw.
In a war, trains are only useful for logistics in areas you already control
This is also part of my point. These lines are, effectively, "uncontrolled" by anyone. They don't have a sizeable military presence, they are peaceful and controlled by civilians. So Russia would, by virtue of being the only ones on it at the time, be in effective control of the line until someone can figure out how to remove them. And if there isn't some established defense against this kind of attack - hijacking the rail network - then it represents an actual risk, not an imaginary one.
Then there's the other point - what do you do once you stop the train? If this train launched from Ukraine into Slovakia, would Slovakia want it to be stopped on their territory, so that their soldiers can be the ones to lay down their lives? Or would they let it continue onward to Czechia - likewise, would Czechia say "This looks like Germany's problem" or try to organise an actual response?