Oil Tank Wagons RTR

Liquids including oil products were carried by the railways from the earliest days. Early wagons conveyed liquids in barrels, however specialist tank wagons were in service by the end of the 19th century. Early wagons frequently had a rectangular tank, but the strength of the cylinder was well known by engineers and specifications for cylindrical tank wagons we included in the Railway Clearing House standards by 1907. These early cylindrical tank wagons had the barrel carried on either cross or longitudinal wood bearers, restrained by raised stops and secured with wire ropes over the barrel and around the top filler.
By the 1930s the diagonal rope securing had been replaced by strengthening and bracing of the end stops, including a longitudinal tie rod between the end stops, though the tank was still held down by strips running between the chassis solebars over the top of the barrel. This is the style of construction adopted by the Air Ministry for the large numbers of new tank wagons needed during WW2.
In the late 1930s a new method of securing the tank to the chassis was being tested. Known as the anchor mounting this comprised a large bracket in the central section of the chassis which fixed the tank barrel to the chassis. Due to the large numbers of 'army surplus' Air Ministry tank wagons available at the end of the war the anchor mounted design did not appear in large numbers until new construction resumed in the later 1940s.
So far almost all oil rail tank wagons were 'unfitted', that is not equipped with a train power braking system, having hand brakes only. While well maintained and usually carrying 'fast traffic' stars these wagons were still limited to trains with maximum speeds of 35-40mph. Meanwhile there had been developments elsewhere which the next generation of oil tank wagons were to benefit from.

From circa 1900 the British railway network had been able to accept axle loads of up to 12 tons (almost) universally. Thus the British railway wagon usually had a maximum all-up weight of of around 22-24 tons, with a load capacity of 12-16 tons. This low, universal axle load limit was extremely restrictive on motive power, so by 1900 almost all except the most minor of minor standard gauge railways had been upgraded to at accept least 15 tons axle load and often 18 tons.

In 1957 a new design of oil tank wagon was built, taking advantage of the increase axle load limits to offer 35-tons glw (gross laden weight) and carried on a modern underframe fitted with vacuum train brakes suitable for train speeds up to 60mph. In this design the cylindrical tank was secured the the chassis by two cross-wise anchor brackets, with wide mounting pads to address the additional stress loads on the tank barrel at this mounting points.
These tank wagons were not to be the standard for long however, as the train-load bulk distribution of oil by rail did not need these wagons to reach destinations on minor branch lines and mainline routes were usually being upgraded to take 20 ton axle loads by the 1920s. Further the elimination of the 'hammer blow' loading from reciprocating steam locomotives following the adoption of diesel traction allowed the civil engineers were quickly agree to loads of first 22.5 and later 25 tons per axle over most of the core network.
The next development was what would become the 'monobloc' design in which the strength of the cylindrical tank barrel forms part of the load bearing structure of the wagon, instaed of being simply the 'load' on a strength underframe. Offering initially 40-tons glw (intriduced in 1962) and later 45-tons glw (from circa 1965) these wagons are well known by their modern era TOPS code of 'TTA' but in the 1960s were vacuum braked. Many hundreds of wagons to this design remain widely used today, the ultimate development being the 'frameless' bogie tank wagons where the tank barrel provides the main structure of the wagon, with the bogies mounted via sub-frames at each end.