Verified answer

The horn blasts are just standard warning signals to persons on or about the track. Signals TO the Sandite operator are warning signs on the side of the track - 3 bars = "Approaching a Sandite site",

2 bars = "Start spraying" and 1 bar = Stop spraying.

I assume you are asking about the UK? As far as I am aware, there are no prescribed horn codes for Sandtite trains - for what reason would the driver need to sound the horn, other than for the same reasons as for any other train ('whistle' boards, staff on the track etc.)? I am pretty sure that the Sandtite/rail cleaning trains that ran on 'my' line had two crew in the cab; a driver and an operator.

Also, they exhibited the same headlamps and marker lights as any other train.