Full Steam Ahead ... malappropriated when applied to Trains?

From the entry in TheFreeDictionary.com:

full steam ahead with all possible energy and enthusiasm full speed ahead The real estate market has heated up, and building is going full steam ahead.

Usage notes: sometimes used in the form it's full steam ahead: It's full steam ahead for Internet service providers today.

Etymology: based on the literal use of full steam in ships, which makes them go at their top speed

So figuratively speaking, you could apply this to anything you like. No rule is broken if you use it metaphorically about things other than ships.

You could use it metaphorically, or you could use the phrase full speed ahead which is more generic and according to google books, both pre-dates full steam ahead and is in more common use.

In answer to the other part of your question, I believe historically, the order would be given by the person in command of the bridge and then transferred by a sort of mechanical telegraph system to an engineer.