Brompton is best in terms of foldability but maybe not in other areas. The biggest problem is it not using standard components, which makes upgrading and customizing limited. For example, I put schwalbe one 23mm (0.9 inch) width tyres on my 20 inch folder, which is a big improvement in terms of speed. The other day I cycled along college road in south London and decided my gear was not low enough so planned to order a Shimano shadow RD and a 12-36 T cassette to lower my gear inch. Things like these can be easily done with Dahon and other folder using standard components. And the 16 inch wheels on brompton are just too small for any serious speed except the road condition is really good. For me 20 inch is the lower limit, 22 inch maybe better but I can live with 20 inch at least in London area.
I understand many people using brompton are not performance oriented but I believe some people do care about performance. One thing is people often compare 1000 £ brompton to 500 £ Dahon or similar. Why not compare 1000£ or even 2000£ Dahon/Tern/Pacific reach etc? Those premium models are not only lighter (under 10 kg, some even under 9 kg), but faster (sleek tyres, top tier road bike components). Some of these can seriously contend with mid ranged road bikes, such as Dahon speed pro TT/vector 27, Tern verge 30h/verge 18x, Pacific reach. The difference with road bikes? They fold in 10 seconds (not Pacific reach) while it takes half an hour to detach a road bike for travelling. These are performance bikes that can fold easily while brompton is a bike for folding.
And regarding the folding, brompton is the best no question (size and looks better). But a Dahon/similar flat bar model is not that bigger. They are similar in thickness and height and only about 10-20 cm longer. If a train is really that packed, a Brompton will cause the same inconvenience as a Dahon/similar (or a suit case). If it's not that packed, both will be fine.