Toronto Hamilton & Buffalo was owned approximately 27% by CP, 37% by NYC proper, and 18% each by Michigan Central and Canada Southern. NYC therefore controlled the company through its own and its subsidiaries’ holdings. The road was independently managed, in part to satisfy fears of a railway monopoly by the City of Hamilton, and in part to prevent conflicts between the interests of the owners, which were generally cooperative wherever they connected. Operating agreements between NYC, CP and TH&B governed through operations between Toronto and Buffalo. CP’s influence appeared greater because CP built many TH&B cars in its own shops, for customs purposes.
IIRC, Canada Southern was owned about 57% by NYC and/or Michigan Central, and the remainder by outside shareholders until CP and CN bought it jointly in the 1980’s. It was under lease to MC.
As far as I know, NYC didn’t actually cross the border at Fort Covington. The CN line crossed to a connection with NYC.