When a company operates, it performs the following activities. Most are optional, but if done they must occur in this order:
* Buy rolling stock(usually optional but sometimes compulsory
* Acquire a minor company from (willing) player, or from phase 5 onwards, from the bidding boxes (major companies only).
* Issue or redeem corporate stock (major companies only).
There are only two steps after train buying: acquire a minor and issue/redeem stock. There is no compulsory train buying step after that. So you have to possess a train before you can acquire a minor.