Me and my brother took the train to l’Alpe d’Huez this year and it was very pleasant, as we drove last time. You can take your bike in a bag on all trains OK, so go from scotland to King’s X, then eurostar to paris, then TGV to Grenoble (3hrs direct from Paris Gare de Lyon). Bus transfer from Grenoble less than an hour. If you book early the prices are very reasonable, and it is a very pleasing way to travel. I left from London at 730am, and arrived in Grenable about 1530, so adding in scotland journey and it should be same day, or perhaps get the sleeper the night before (my brother did this from Glasgow)..