Short answer: Exeter, Plymouth, Dawlish, Teignmouth, Paignton and Totnes are all on mainline railways. Barnstaple is the end of the Tarka Line from Exeter. From those, ferries and the Tarka Trail get you to places that look car-only on a map.
The main line runs right along the coast between Exeter and Newton Abbot, which puts two seaside towns on the network with stations more or less on the beach. Add the branch lines, the ferries and thirty miles of traffic-free trail and a car-free trip is genuinely workable.
Dawlish station is right on the seafront with the sea wall promenade running both directions from it — this is the single easiest car-free day out in Devon. Teignmouth station is a five to ten minute walk from the front and close to the Back Beach and the Shaldon ferry.
Both sit on Brunel's sea wall line, so the journey itself is part of the day.
This is what makes the south coast work without a car. From Kingswear the ferry takes you into Dartmouth. From Dartmouth, boats run down to the castle and upriver towards Totnes. From Teignmouth the ferry crosses to Shaldon. At Salcombe, the passenger ferry reaches the best beaches without the forty-five minutes of lanes a car would need.
The Tarka Trail is thirty miles of flat, paved, traffic-free former railway from Barnstaple down the estuary through Bideford and beyond. Hire a bike at the Barnstaple end and Instow, Appledore and Bideford are all reachable. No mud, benches throughout, and refreshment stops in both directions.
The 173 stops directly outside Castle Drogo on Dartmoor, which saves the narrow lanes. Sandy Bay at Exmouth is most simply reached by the bus that stops nearby rather than through the holiday park. Combe Martin, Ilfracombe and Woolacombe are linked by bus along the north coast.
Most of Dartmoor, the Hartland peninsula, Clovelly, and the smaller South Hams villages. Croyde and Woolacombe are reachable by bus from Barnstaple but the timetables are thin out of season.
Pick a town and plan the day around who's with you.
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