The mass-produced US Sherman tanks were too high to fit within the British loading gauge using the WW1 era 'Rectank' flat wagons, though could be accommodated on bogie well wagons, normally used as boiler trucks. These wagons were designed for loading by crane, not for carrying self-propelled vehicles, so lacked end loading ramps. The railway company engineering departments already had a suitable style of wagons used for carrying steam road rollers, but as the railways only owned a handful of rollers these wagons were few in number, while the War Department needed hundreds and rapidly!
The War Department design tank carrier was designated Warwell and built along with it's flat deck companion the Warflat for trucks and jeeps by several of the private wagon building companies including Gloucester RCW, Head & Wrightson and the Southern Railway workshops. The wagons were designed for end loading and to allow tanks to be driven along the train with a well load capacity of 50 tons spread across at least 14 feet of the bed. To relieve load on the bogies during loading screw jacks were fitted under each headstock to support the weight directly against the rails below. Diamond frame bogies were fitted as these were simple to construct and a well-understood design for maintenance.
The wagons were spread between the four mainline railway company's, though owned by the War Department and merged into British Railways stock at Nationalisation, often with a regional prefix from their previously allocated owners. To make use of the 50 ton capacity many were fitted with bolsters for carrying steel and rail, both as traffic wagons and engineering wagons. Meanwhile the engineers fitted many appliances to their wagons, including using them as a chassis for twin-jib track panel cranes and cranes suitable for off-loading long continuously welded rails. Reference to Paul Bartlett's site will provide many more examples of Warwells in use carrying engineers loads, steel sections, and even other wagons!
A number of Warwells remain in service today. The Ministry of Defence fleet was upgraded in the late 1970s with new Gloucester GPS bogies featuring roller bearings and air train brakes, making the wagons suitable for running in 60mph Speedlink trains. Many of these wagons are still available today, just about able to accommodate a Warrior AFV within the British loading gauge. As the engineering departments of the national railway network have invested in new equipment several heritage railways have purchased withdrawn Warwells and crane-equipped Warwells for their own engineering teams, so these wagons will be around for many more years to come!