Are you travelling as a small group? Best choose the option of a private transport company, as that can prove much simpler and cheaper than the train. It’s not necessarily a service one thinks of automatically, but several bike-hire companies and taxi companies can offer transfers of people + their bikes.
The platform www.marandofacile.com can organize this service not just for small groups but also for people travelling alone. Many other services centred around bicycle tours and hiking are available via this platform, created by the travel agency Visit Ouest.