A Mallet Garratt

This is an artist's impression (taken from this web site) of a proposal for a 3' 6" gauge locomotive for South African railways, produced in 1927 by Beyer Peacock of Manchester, though nothing came of it. What sets it apart from other Beyer Peacock locomotives of the Garratt type is that it's actually four, rather than two, engines fed from a single boiler; two high pressure and two low pressure engines of two cylinders each, the low pressure engines articulated to the high pressure in the Mallet fashion.

The few comments about it that I've seen around the web condemn the proposal as impractical, largely because the only other attempts to feed more than two sets of cylinders from one boiler were abject failures. The difference between the Beyer Peacock Mallet-Garratt and the triplex articulated locomotives of the Eerie Railroad and the Virginian Railroad (see both types here) is that the latter fed two sets of low pressure engines from one high pressure set of cylinders yet exhausted only one set of low pressure cylinders through the smoke box, thus halving the available draught. It's also significant that the Eerie Mallets were modified very shortly after building to accommodate larger fireboxes, which still seem to have been too small.

I think the locomotive could have been very successful, because both of its constituents were very successful. I'm aware that the conclusion does not necessarily follow but the reasoning seems simple enough: The Garratt fed two high pressure engines, just as the later US simple expansion articulated locomotives (eg Big Boy) did, without any trouble and the Mallets simply used the exhaust steam from one engine to drive another.

If a (high pressure steam)+b (low pressure steam) works:


and 2a works:


then 2(a+b) must work:


Mustn't it?

A standard gauge version was also proposed for SAR but that too came to nothing, sadly. I can't help wondering what might have been had Alco produced something of the type under the North American licence held by them. It isn't such a wild flight of fancy.

You can see the picture above, with general arrangement drawings of both the narrow and standard gauge proposals, here.