The 5AT locomotive, excluding the tender, is based on the size and format of the British Railways Standard Class 5MT 4-6-0 design of 1951. The reasons for this are as follows.
- Given the power/weight ratio now possible in steam traction, it is an appropriate size of locomotive for the intended duty of main line charter train service.
- The deep firebox of a 4-6-0 has, size for size, a higher evaporative capacity than a shallow firebox boiler, and is ideally suited to burning oil or coal (using the Gas Producer Combustion System).
- Basing the design on an existing one will significantly reduce the design complexity, time and cost. All overall dimensions constrained by the moving structure gauge being kept within those of the BR Class 5MT will facilitate route acceptance.
- A modest size locomotive allows use of a large tender without exceeding the permissible length for turning facilities, thus maximising its operating range.
- The route availability is high - it is a 'go-anywhere' type.
- The relatively small taper boiler gives good forward visibility from the cab, a factor of paramount importance to safe operation at high speed.
- The 4-6-0 is the quintessential British locomotive type, for it was in Britain that the 4-6-0 was developed more than anywhere else, and the Class 5 may be considered to be the quintessential 4-6-0.
For these reasons it is considered most appropriate to base the 5AT design on the general weight and outline of the BR Class 5MT.
The image above compares the 5AT with the 5MT. The size of tender is the most striking difference, and the feature that gives the 5AT a much greater operating range.
An graphical comparison of performance between 5AT and 5MT is shown below.