Cycling in the high mountains. Alps, Dolomites, Pyrenees etc

TGV

Bikes go for €10 on some TGV & Intercité trains, advance reservation required: Some high-speed TGV trains & some Intercité trains have special bike spaces and will take bikes for a fee of €10 if you book a bike space in advance. This includes some TGV-Atlantique (Paris-Brittany, Paris-Bordeaux-Biarritz-Lourdes), all Paris-Limoges-Cahors-Toulouse & Paris-Vichy-Clermont Ferrand Intercités, some TGV-Nord (Paris-Lille), some TGV-Est (Paris-Reims-Strasbourg). There are a maximum of 4 bike spaces per train, on the routes and trains that offer this feature.

Overnight

Bikes go for €10 on some French overnight trains: Some Intercités de Nuit overnight trains within France take bicycles in a special bicycle compartment if you reserve space in advance and pay a small fee (€10-€15), while you sleep in a 2nd class couchette (it won't let you add a bike if you book a 1st class couchette!). There are just a handful of Intercités de Nuit routes, routes that take bikes include Paris to Toulon, Cannes, Antibes& Nice, Paris to Toulouse, Paris to Lourdes & Tarbes, Paris to Perpignan, Collioure, Port Vendres & Cerbère. But an afternoon Eurostar then an overnight couchette train a good way to get your bike from the UK to southern France. You'll have to cycle across Paris, though.

https://www.seat61.com/bike-by-train.htm#France