A 1950s comedy film produced by Ealing Studios set on a fictional British branch line between Titfield and Mallingford which has been condemned for closure by British Railways. Sam Weech, the village vicar, and Squire Gordon Chesterford, the grandson of the founder of the branch line, decide to buy the branch line and operate it themselves, with the help of a sizable donation from Walter Valentine, a wealthy regular at the local pub.
This plan is bitterly opposed by the local bus company, who were looking forward to a monopoly with the closure of the railway and call those wanting to run the railway "amateurs", they are also viewed with some trepidation by George Blakeworth, the town clerk.
After an inquiry where both sides put forward their arguments for and against the railway being run by the amateurs, those wishing to run the railway are granted month's trial period before a railway inspector comes to decide if they can sufficiently run the railway.
Weech puts himself in the position of Driver, Valentine puts forward Dan Taylor, a retired railwayman, for the position of fireman and Chesterford takes up the position of guard.
Despite the bus company's attempts at sabotaging the railway's work, Including blocking a crossing with a lorry full of bricks and shooting holes in the water tower, the railway becomes a huge success, and so the bus company offers a merger between them and the railway, which is outright refused so the bus company derail the branch line's only engine and carriage on the eve before the inspection and the town clerk is mistakenly arrested for derailing the engine after trying to stop the runaway train before it crashed.
Valentine and Taylor get drunk and take a pump trolley to Mallingford to steal another engine for the railway. This plan goes awry when they drive the engine through the town across a park and into a tree, resulting in their arrest.
Meanwhile, Weech figures out where they can get an engine from for the inspection and rushes to the police station to release the town clerk. As they are [[BreakOutTheMuseumPiece wheeling the old engine out of the museum]] Weech realises that they haven't got a carriage to which the clerk says that Taylor can help them. So they they take Taylor's home, an old railway carriage.
The next morning the Inspector arrives to see the ensemble the railway has put on, but not before Ollie Matthews, The Bishop of Welchester, a fellow railway enthusiast and an old friend of Weech's, shows up for a visit and is drafted in as fireman. They are told by the local handyman that he's had to use rope to connect the engine to the flatbed because of the incompatible couplings between the two. Because the engine only has a weak handbrake if they stop the train suddenly using the brakevan's stronger brake the weight of the whole train would be put on the rope connecting the flatbed to the tender.
The lorry taking Taylor and Valentine to prison crashes with the bus from the rival bus company, the owners of which assume they've been rumbled causing one of them to admit their crimes.
Back at the station the train starts to leave but is stopped and commandeered by the police to take them to Mallingford with the prisoners, now consisting of Valentine, Taylor and the two bus company owners. The train then continues until the inspector decides to test the emergency brake and the rope snaps, leaving the carriage and brakevan at a standstill and the engine continuing onwards. Chesterford ropes in several locals to help push the stranded train down the hill to catch up with the engine which had come to a halt by the now repaired water tower, on the way they borrow a set of chains from the steamroller they jousted with, to replace the rope.
Once the train is recoupled to the engine they set off again, feeling there is now no hope of them making it on time after all the delays. They arrive at Mallingford ten minutes late and feel all is lost. The inspector informs them that actually, they only just passed (and almost failed because their average speed was almost over the limit). The villagers celebrate and the other drivers at the station blow their engines' whistles in celebration of their Success
The story is based on the preservation efforts of a group of people that went on to save the Talyllyn Railway, which could be classified as a [[CoolTrain Cool]] ''[[CoolTrain Railway]]'', entirely worked, preserved, and funded by volunteers and indeed the first preserved railway in Britain.
!!This film provide examples of:
* AccidentalPublicConfession: When Pearce and Crump crash into the police van, Pearce blurts out, "We didn't do it, I tell you!" The policeman's response? "Didn't do what?"
* ArsonMurderAndJaywalking: The long list of charges against Valentine and Taylor, amounting to stealing a locomotive and driving it through a town, including 'being drunk and disorderly,' 'driving an unlicensed vehicle on the highway,' 'defective tyres' and other minor offences.
* BreakOutTheMuseumPiece: The Titular Thunderbolt.
* ChekhovsGun: Sam's postcard of the Thunderbolt, Dan's railway carriage home, Sam's warning about the rope coupling.
* CloudCuckoolander: Mr Valentine
* CoolTrain: Two of them; The first is a generally normal-looking tank engine, a BR 14xx Tank engine to be exact, used to joust against a steamroller. Not forgetting the titular Thunderbolt. It should be noted that Lion, the engine that played Thunderbolt, was well over 100 years old at the time the film was made.
** The air-smoothed express train seen briefly at the beginning is pretty cool, too.
* DrivesLikeCrazy: How about driving a railway locomotive along the road?
* EnemyMine: Harry loathes the railway and refuses to provide any assistance... until Miss Hampton promises to marry him.
* FreeWheel: When the railway's first locomotive is derailed.
* LoveInterest: Miss Hampton to Harry.
* NiceJobFixingItVillain: Pearce and Crump's sabotage serves to wreck their lorry, to put Harry's steamroller out of action and to get lots of publicity for a tiny little country branch line.
* RailEnthusiast: Vicar Sam Weech and Ollie Matthews, the Bishop of Welchester are both this. Real life railway enthusiasts tend to be big fans of this film, as in addition to its CoolTrain content, it's perhaps the best and most affectionate portrayal of train fandom.
** Meta example: T. E. B. Clarke, the screenwriter, became a RailEnthusiast in the course of researching the film.
* ReasonableAuthorityFigure: The Ministry official is stern and humourless, but ultimately gives the line his blessing. Mr Blakeworth isn't a fan of the line at first, but comes around. The Bishop of Welcester is less concerned with reprimanding his absentee vicar than with getting his hands dirty on the engine.
* RippedFromTheHeadlines: As with many Ealing comedies, this film targets the widespread closure of smaller lines by British Railways and the increase in road traffic. It also deals with the then-novel concept of preserved railways.
* ShownTheirWork: The film almost completely avoids JustTrainWrong.
* StrawmanPolitical: The Union official seen briefly at the town meeting, who's determined to stop all worker exploitation whether it actually exists or not.
* StreisandEffect: The line gets lots of negative publicity due to the bus company's sabotage efforts, and as a result becomes so well known that it becomes one of the only profitable branch lines in Britain.
* SuspiciouslySpecificDenial: "We didn't do it! It wasn't us!" At this point, nobody had accused them of anything...
* ThrowItIn: The "sharp pull-up" actually damaged the tender of the locomotive playing the Thunderbolt, but it looked great on film.
* UnintentionalPeriodPiece: In 1952, this was a topical film on the subject of modernisation. Today, even the "modern" aspects of life in Titfield are charmingly dated.
* VeryLooselyBasedOnATrueStory: Although inspired by the real life Talyllyn Railway, in reality very little beyond the basic concept of volunteers taking over a closed railway made it to the screen.
* TheVicar: Reverend Weech, and to a lesser extent the Bishop of Welchester.