//Since 2015 the “Scotrail” franchise has been in the hands of Abellio.// indeed, and the Scottish government themselves tendered and let the franchise. the issue is cross-border services, with franchises let by a rump uk government over territory of another sovereign power. already the scots are spoiling for a fight because they have only a consultative role in who gets to operate services from England into Scotland.
another issue is regulation. with Scotland independent, there will be 2 ORRs for train companies to deal with, and 2 regulatory frameworks to comply with, which will increase costs.
at present, train operators can cross-subsidise anglo Scottish services from profitable London to "northern powerhouse" destinations; if independence rules that out, cross border services might end up being unsustainable.