Might I mention the Brighton Main Line here?
Five years ago there were no less than six different TOCs operating on the route, most of which catered for different markets. FGW gave passengers to the west of the route a link to Gatwick, VXC ran long-distance services avoiding London, and GX gave passengers travelling to Gatwick (!) a special express service from London, while SE concentrated on the route east of Redhill and gave it links to Gatwick (and Horsham) to the south and London Bridge to the north. This left only SN and FCC, who for the most part serve different markets on the route, with FCC covering both locals and fast services to London Bridge and SN covering routes diverging away from the line, as well as the Brighton Express. Thus, most of the competition was between SN and FCC - the former of which was operated by the latter's former operator.
Now, two of those no longer serve the route, and a third has been swallowed up by one of the others, and in the future the main competitors are set to merge leaving only the SN/FCC giant against FGW, which only runs Redhill to Gatwick non-stop (and then only hourly) anyway...