This would be conventional carriages behind a class 73 locomotive. It would go from London to Weymouth round the coast.
Out late on a Friday night calling at:
London Victoria > Denmark Hill [3] > Lewisham [4] > Welling > Dartford > Gravesend > Strood > Rochester > Chatham > Gillingham > Rainham > Sittingbourne [2] > Faversham [3] > Whitstable > Herne Bay > Birchington-on-Sea > Margate [1] > Ramsgate [2] > Sandwich > Deal > Dover Priory [1] > Folkestone Central > Ashford [6] (reverses & switches to diesel mode) > Rye > Ore (switches back to electric mode) > Hastings [2] > St Leonards Warrior Square > Bexhill > Eastbourne [3] (reverses) > Hampden Park > Lewes [4] > Falmer > Brighton [3] (reverses) > Hove [3] > Portslade > Shoreham-by-Sea > Lancing > Worthing [3] > Durrington-on-Sea > Angmering > Bognor Regis [2] (reverses) > Barnham [3] > Littlehampton [1] (reverses) > Chichester > Havant > Hilsea > Fratton [2] > Portsmouth & Southsea [3] (reverses) > Fratton [1] > Cosham > Portchester > Fareham > Swanwick > Woolston > St Denys [4] > Southampton Central [4] > Totton > Brockenhurst [3] > New Milton > Christchurch > Bournemouth > Poole > Dorchester South > Weymouth [1] (reverses) > Upwey (switches to diesel mode) > Dorchester West > Yeovil Pen Mill > Castle Cary [2] > Bruton > Frome > Westbury [3] > Trowbridge > Bradford-on-Avon > Bath Spa > Bristol Temple Meads
Returning on a Saturday night calling at:
Bristol Temple Meads > Bath Spa > Bradford-on-Avon > Trowbridge > Westbury [2] > Bruton > Castle Cary [2] > Yeovil Pen Mill > Dorchester West > Upwey (switches to electric mode) > Weymouth [1] (reverses) > Dorchester South > Poole > Bournemouth > Christchurch > New Milton > Brockenhurst [2] > Totton > Southampton Central [1] > St Denys [3] > Woolston > Swanwick > Fareham > Portchester > Cosham > Fratton [2] > Portsmouth & Southsea [3] (reverses) > Fratton [1] > Havant > Chichester > Barnham [3] > Littlehampton [1] (reverses) > Bognor Regis [2] (reverses) > Angmering > Durrington-on-Sea > Worthing [2] > Lancing > Shoreham-by-Sea > Portslade > Hove [2] > Brighton [3] (reverses) > Falmer > Lewes [3] > Polegate > Eastbourne [3] (reverses) > Bexhill > St Leonards Warrior Square > Hastings [3] > Ore (switches to diesel mode) > Rye > Ashford [6] (reverses & switches back to electric mode) > Dover Priory [2] > Deal > Sandwich > Ramsgate [3] > Margate [3] > Birchington-on-Sea > Herne Bay > Whitstable > Faversham [2] > Sittingbourne [1] > Rainham > Gillingham [2] > Chatham > Rochester > Strood [2] > Gravesend > Dartford [2] > Welling > Lewisham [3] > Denmark Hill [4] > London Victoria
This journey will undoubtedly be very long, but that's kind of the point. It's to give clubbers & low-budget tourists who have been out late at night a place to get some sleep without having to pay for a hotel/hostel.
Perhaps first-class passengers could ask to be woken up when they reach their destination (would make more money by encouraging more people to buy first-class).
Based on National Rail Enquiries searches, this service would take 10 hours 30 minutes if it were to go all the way to Bristol, or 8h33m if it were to terminate at Weymouth.