At the moment, the trains aren't running, as the lines are blocked. First Great Western are expecting the disruption to continue until Dec 29th. It's worth checking their website for updates. There is a bus service running from Exeter to Barnstaple. It's okay at the moment, but could be affected if there is more flooding on the road. Hopefully, by the time you travel this dreadful weather will be behind us and you will be able to reach Exeter easily.