The Dapol O Gauge TTA tank wagons will feature:
- Highly detailed and accurate body shell and chassis with many separately added details, including etched hand rails, ladders and walkways
- Metal Instanter coupling chain and sprung coupling hook
- Metal sprung buffers
- Compensated chassis, along with a weight in the tank barrel and finely profiled metal wheels lead to extremely smooth running
- Variations and modifications are tooled
- Type A & Type B vehicles to be modelled
Through the first half of the 20th century oil tank wagons were among the heaviest wagons in general service with a payload up to 20tons and fitted with hand brakes only. Wagons over this weight would be subject to restrictions on the routes and destinations they could reach.
Routes with the most restrictive axle loadings were the most minor of byways and could easily be served by road transport from a nearby mainline station. Esso were the first to introduce a larger wagon, purchasing a small fleet of long-wheelbase tankers with a capacity of 35tons and equipped with vacuum train brakes; the subject of the well-known Airfix (now Dapol) OO kit.
Around this time British Railways was 'modernising', replacing steam locomotives with diesel traction and seeking to remove the 'unfitted' trains from the network. Alongside this more scientific approach to assessing infrastructure revealed that axleloads up to 22½ tonnes (BR now using metric tons) could be accepted over almost the entire core network, allowing a maximum laden weight of 45 tonnes from the early 1960s.
The Beeching reports of the early 1960s also changed the focus of British Railways operations and the large oil companies were offered lower rates trainloads of product operating between two terminals on the basis of a minimum annual tonnage guarantee and modernisation of the wagon fleet.
The oil companies took advantage of these changes, quickly ordering a fleet of 45-tonne glw (gross laden weight) 4-wheel tank wagons, all equipped with train brakes, initially vacuum but changing to air braking as more air brake equipped diesel locomotives came into service. The introduction of the TOPS (Total Operations Processing System) computer based traffic and wagon management system in the early 1970s saw the arrival of the familiar TOPS code of TTA for the air braked 45-tonne tank wagons, with TTV for the remaining vacuum braked examples.
Despite the later development of bogie tank wagons with laden weights of up to 102 tonnes the 45 tonne 4 wheel tanks have remained in widespread use as for several oil products they offer a better ratio of load to tare weight than the bogie tankers.