German here. The idea in general is extremely awesome... but the problem is, us Germans (or rather, our previous Conservative government) took pride in investing in highways instead of railways - and so, when the beautiful tunnel is built, it will take many years until it can be actually used at capacity because we couldn't be arsed to build out the railway route 5702 from Kufstein to Rosenheim and 5510 from Rosenheim to Munich [1] to four rails instead of the current two, which are split between regional S-Bahn trains, inter-regional trains, freight trains and high-speed trains.
[1] https://www.deutschlandfunkkultur.de/brennerbasistunnel-deut...