The 'Monobloc' tank wagon appeared on the railways in Britain in 1958. As its spelling suggests, it was based on a French design. By lowering the tank between the solebars the centre of gravity was also lowered, thus allowing the accommodation of a larger tank. Wing plates welded to the solebars hold the tank in place.

'A' class tank wagons have internal stays to strengthen them. Another difference is that 'A' tanks usually have ladders on the end of the tank but 'B' class tanks usually have them on the side. As the 'B' type carries heavy oils, it has internal heating to facilitate unloading. The 'Monoblocs' have a 15 foot wheelbase and can carry 45 tons.

As the post-war tank hire business began to get underway, the hire company's logo started to appear on tanks and then, in the mid 1970s, petroleum companies began to remove their own identification.