The incident is the fourth to affect the ScotRail-run service this week, according to posts on Twitter.
The train operator said 125 passengers on the Inverness to London Euston train were switched to taxis and other trains when it arrived at Watford around 10:45am - nearly four hours late.
A locomotive hauling the train broke down at Rugeley in the West Midlands and a replacement engine had to be sent from Euston.
By the time the train reached Watford there was no platform long enough available at Euston.
The train, which also includes carriages from Aberdeen and Fort William, is a quarter of a mile long - one of the longest in Britain. It uses locomotives provided by a separate firm, DB Schenker.
On Monday, ScotRail tweeted that the Glasgow to London sleeper had been delayed at Milton Keynes by a “fault”, when it was running more than two hours late.
On Tuesday, former First Minister Lord (Jack) McConnell tweeted: “ScotRail fail meant a grumpy end to a good day yesterday. Second week running no sleeper train ‘buffet’ car. Poor show and bad communication.”
On Wednesday, film maker Anthony Baxter tweeted about the “heavily delayed” London to Aberdeen sleeper.
A ScotRail spokesman said he was unable to provide comments on the incidents today.
Aberdeen-based FirstGroup, which runs the service, last month lost a 15-year franchise to operate it from next April to rival Serco.