Depends whose figures you use, The LMS and especially the LNER had been affected by the 1930's depression due to them serving the more industrial parts of the Country, the Southern was predominantly a passenger carrier before the age of mass car ownership and because of geology did not make extending tube lines south of the river easy had little competition from them compared to North London, one tube made it to Morden and then the Southern got an agreement that no more would come south. it built the line to Chessington as late as 1939 which indicates it was still investing. It should have continued to Leatherhead but WW2 and then the green belt stopped that part being finished. The Great Western served enough wealthier parts of the country that it wasn't as badly affected by the depression even though a good part of its freight was coal from the South Wales area.
The latter two were beginning to make a modest making a profit again and the LMS, at the time the largest corporate body in the world made a reasonable amount from its hotels and shipping operations. What isn't in doubt that the wartime traffic and lack of maintenance would have needed a lot investment to remedy, and it has been suggested the government of the day found it easier to nationalize rather than pay it's bill. The Southern and GWR boards were against nationalization, Ironically one of the GWR directors was Harold Macmillan under whose government in later years Dr Beseeching was brought in to sort out the railways.
The Nationalized British Railways did make an operating profit till
1955 , ironically just as it went into an operating loss the government poured £1.2 Billion into modernizing the network, unfortunately a lot of it was wasted as to many entrenched in the industry modernization meant replacing old steam locos for diesel ones and old wagons for new ones without addressing the main issue that the freight that they would have carried had transferred to road and far too many places still had two routes serving them when one would have sufficed, this poor treatment of such a large investment put following governments off investing in the railways to such an extent ever since.Arguably if the Railways had remained as private companies they may not have splashed money quite so freely on lost causes,closed lines a decade or two earlier than the Beeching cuts and moved traffic where necessary to the road interests they already had shares in a process they had started before WW2.
G.Harman