The accidents are the same as if you bump into someone while riding a bike. It would also be so ridiculously safe and easy, that you'd just need to tell kids that leaning forwards makes you go forwards, backwards = backwards, etc.
And no licenses or medium ages would really be necessary
So even kids could travel in the trenches.
There could be tracks on top of it, with room to get down, for cargo transport
Yeeeahhhh, no. Cargo transportation would be impossible, it would be expensive as all hell to maintain it, it would ruin a large sum of industries, it's not generally as easy as it seems. Maybe you should do a tad more research into your topic before making such massive claims. Thanks.