An extremely important aim of the 5AT Project is to ensure the continuation of steam locomotive development. Indeed, it will be vital to the ongoing viability of any commercial enterprise established to develop the 5AT that there will be an ongoing development program.
In preparing the Feasibility Study for the 5AT Project, the 5AT Group has looked at several possible locomotive developments evolved from the 5AT design that could take place in the future. The most immediately attractive of these are an 8AT 2-8-0 that could be used for heavier haulage than the 5AT and an 8ATT 2-8-4T that might prove viable for tourist and independent railways. Both these projected locomotives would incorporate components of the 5AT while omitting some of the more refined aspects of the design required for high speed operation.
For the longer term, one of the Appendices to the Feasibility Study looks in detail at the possibilities for developing a steam locomotive that matches the performance of the ubiquitous EMD Class 66. The proposals include the use of 6 powered axles mounted on articulated frames though otherwise bearing no resemblance to Bulleid's ill-fated Leader steam locomotive design of the late 1940s.
The Group do not rule out the possibility that the 8AT, 8ATT or other developments might make a better business proposition than the 5AT and as such might be the initial SGS locomotive demonstrator.