Why might a large railroad be better organized as M-forms than U-form Organizations?
Even before the first metals and manufacturing companies organized in the US and in Germany, US railroads in the nineteenth century were M-form organizations based on geography.
In general, the larger a firm is, the more likely it is that it should be organized as an M-form firm.
In the case of a large railroad, the company will be big enough and have enough different functions that it would be better for it to organize as an M-form. There might be different divisions based on geography. Alternatively, there might be different divisions for things like sales and track maintenance and scheduling. These divisions would be complementary but would not need to be contained within one organization. This would have the advantage of having a great deal of decentralization of command. It would allow for each division to move more rapidly and make decisions more quickly.
Because a big railroad is such a large firm with so many functions, it would be better to organize it as an M-form.