Well yes it's on the roadmap so that means it's being worked on. So when raildriver support start's you should buy it
To add to what's been said, Raildriver support is "In production," which means it's actively being worked on. You can keep an eye on the roadmap for its status to advance (to show when it's soon to be released). Here's a link to the current roadmap: https://live.dovetailgames.com/live/train-sim-world/articles/article/tsw2-roadmap-1-september The roadmap is updated every two weeks (on a Tuesday), so a new update will be available tomorrow!
If DTG approach this intelligently, in theory no specific support for any controller will be needed. TSW2 will hopefully be able to bind any button or axis recognised by Windows to an appropriate in-game function. That way you will be able to use any device you wish, but I would like to see DTG confirm this.
The Frequently Asked Questions mentions all forms of DirectInput devices: - Does work on Raildriver come with any other controller options? Raildriver is just one option. We will work to implement all forms of DirectInput devices. It is likely an API will be released for more customized controllers.
MY advise don't buy one just yet. wait for it to be added to the game, let those who already have one test it with the game and to see how it works, see what feedback they give and then decide if you still want to get one or not
Yes TSW2 will support the raildriver as it is on the roadmap, I myself finally took the dive and spent big money on getting mine.