What the journey looks like
The Cardiff service runs on the Great Western main line and crosses into Wales through the Severn Tunnel, a 4-mile (7km) bore under the Severn Estuary that was, when it opened in 1886, one of the longest underwater tunnels anywhere in the world. It's fully electrified track and modern rolling stock the whole way, and the direct service means no changes and no platform-hopping — you board at Paddington and get off in the middle of Cardiff.
There's a short stretch either side of the tunnel where the line runs close to the Severn estuary itself, which is worth a window seat if the tide and light cooperate, but the main appeal of this route is simply how uneventful it is for something that ends in a different country.
Why Cardiff earns the detour
Cardiff is the capital of Wales, and it shows from the moment you step off the train: station and street signage is bilingual, Welsh first, and the language is genuinely spoken around the city rather than being a museum-piece. Cardiff Castle sits in the middle of the shopping district, a Norman keep with a Victorian Gothic Revival mansion built into it by the eccentric 3rd Marquess of Bute.
The city has spent the last three decades reinventing its waterfront: Cardiff Bay was once a coal-exporting dock serving the South Wales valleys, and is now a mixed leisure and residential harbourfront with the Senedd (the Welsh Parliament building) and the Wales Millennium Centre arts venue. The Principality Stadium, home of Welsh rugby, sits right in the city centre on the banks of the River Taff — on a match weekend the whole centre reorganises itself around it.
Planning the day
At under two hours each way, Cardiff works comfortably as a long day trip from London, though it rewards an overnight more than most of the other short routes on this list simply because there's enough in the city and its surroundings (the Bay, the castle, the valleys beyond) to fill more than an afternoon. Advance single tickets are usually the cheapest way to book if your dates are fixed; turn-up-and-go off-peak fares are the more flexible but pricier option.
No seat reservation is required for a journey this length, and the train has standard luggage racks rather than the more limited overhead space you'd find on shorter suburban services — fine for a weekend bag, less fine for anything you'd need to check on a flight.