Köln - Bruxelles (130 miles aka 209 km). Don't get me started about the incompetence of DB» .
Yes, I've done that too, in the other direction. There was a Belgian rail strike. I was going to Aachen - the bus stopped at a service station outside Aachen (which was no good to me), so I ended up going into Koln and getting a train back!
In the
UK▸ , the longest for me is Plymouth to Bristol, a few years ago now.
Are you following me around as a sort of reverse doppelganger? Our Köln - Bruxelles journey was also due to a Belgian rail strike, in 2014, which DB should have told us about. Ended up with a night in Bruxelles.
But I have also done Bristol - Plymouth by bus.
For diversions, I've done Glasgow - Birmingham via Newcastle to avoid a coach to Carlisle, provided because the wires were down over Beattock.