The Amtrak engineers usually blow "two to go" as an acknowledgement to the radio transmission from their conductor that station work is complete and they are okay to depart....in absence of an operating communication buzzer.
On the NEC, conductors will use either the radio or communcation buzzer to signal engineers to depart; I've never heard them use the horns for that purpose on the NEC....yet.
Outside the NEC, using my last trip to Pittsburgh as an example, the crew just relied on radio communication for the "two"; the horn was not used. Then, from Harrisburg on west, the engineer did use his horn to acknowledge the conductor's "two".
And more recently, on a trip to and from Toronto on the Maple Loaf, the horn wasn't used at all upon departures from stations (I was seated in the coach right behind the locomotive both ways).
"CSX Detector. Milepost Six Point Four. No Defects. Repeat: No Defects. Total Axles Seven Four Seven. Detector Out"