As Tim O'Connor has already noted, tank car dome capacity provides a space for thermal expansion of the lading in order to prevent over-pressurization and/or lading leakage. Most cars are based around petroleum products which uses a factor of 2% – so an 8,000 gallon car requires a dome capacity of 160 gallons, a 10,000 gallon car requires a 200-gallon dome and so on. Traditionally the major use for tank cars is petroleum (and related products), and most cars utilize this 2% number.

Other commodities have very different requirements: for example, acid has much lower expansion numbers, so acid cars tend to have very small domes (which are also frequently much narrower than standard domes, although limited by the need to provide access to the car for workers). There are some other cars with very large domes, which were provided for a very specific purpose.

The expansion requirement has not gone away, but today the understanding is that this expansion capacity is to be provided within the tank body itself – the cars are simply not filled to the top. The reason for this change is simplified construction and a stronger carbody.

Ian Cranstone
Osgoode, Ontario, Canada
lamontc@...