Whistle and hand signals were the main means of communication before two-way radio. Often, the engineer would alert the crew in the caboose using the whistle. In the case of a pusher, whistle signals were used by the pusher to indicate it had coupled to the train, and the lead engine would answer in acknowledgement.
As far as coordinating speeds, remember, track speed isn't like speed limit on the highway. If the highway speed limit is 60, that means you can't exceed 60. If the track speed is 60, the train is supposed to do 60. The schedule demands this.
Then there are cab signals, which actually date back to the 1930's. If a railroad has cab signals, all engines must be equipped to accept them. The cab signal constantly tells the engineer the current track speed.