On RMWeb, there’s a thread on 1923 Grouping. While the initial post was about the reasons behind the grouping (it was an alternative to nationalisation, which was seen as too radical a step in 1923), it’s spawned a side-discussion on alternatives.
For example, what if, rather than the government imposing the big four, they’d instead changed the law to make it much easier for railway companies to merge, and let the market decide which groups would form? We probably wouldn’t have seen the uncomfortable forced marriage between the LNWR and the Midland, for starters.
One scenario that sounds interesting would be a merger between the Midland and the LSWR, forming a network stretching from the Scottish border (and probably beyond, as a Midland-GSWR merger would be likely) to Cornwall. To counter that, the Great Western might merge with the Great Central to form a rival network with a very similar national footprint. This gives a couple of big company networks that will be very different from any of the historical ‘big four’. I wonder what they might have looked like? Would either of them have gone for large-scale electrification?