The route
Penzance is the far end of the Great Western main line, and this direct service is, by scheduled time, the longest same-day journey on the entire British mainline network — longer than London to Edinburgh, despite feeling like a more modest domestic trip on the map. The gap between the fastest and typical times reflects the number of stops most services make along the length of Devon and Cornwall.
The most distinctive stretch of the whole route is the sea wall at Dawlish in Devon, where the track runs directly along the coast, at points almost over the water itself, with waves breaking against the wall a few metres from the carriage windows at high tide. It's one of the most photographed stretches of railway in Britain, and also one of the most exposed — a storm in February 2014 breached the sea wall entirely and severed the line for two months, a reminder that this section of track sits unusually close to the sea for a mainline railway.
What's at the end of the line
Penzance is the westernmost mainline railway station in England, and it functions as the gateway to the far tip of Cornwall: St Michael's Mount, a tidal island crowned by a medieval castle that you can walk to across a causeway at low tide, sits just along the coast at Marazion. From Penzance, onward ferry and helicopter or small-plane connections continue to the Isles of Scilly, an archipelago with a noticeably milder, almost subtropical climate for Britain.
Cornwall's identity is distinct from the rest of England in ways similar to Wales and Scotland — its own Celtic language (Cornish) survives in place names and a small revival movement, and the coastline, fishing villages and mining heritage (much of it now UNESCO-listed) give the far southwest a character that's genuinely different from anywhere else reachable this directly from London.
Why to treat it as its own day
At nearly five hours each way on the fastest service, and closer to five and a half on a typical one, Penzance is not a journey to bolt onto another destination the same way Bath or Cardiff can be. Building a Cornwall stop into an itinerary works best as its own multi-day leg, with at least one overnight, rather than an ambitious add-on to a trip centred somewhere else.