An ammeter is always installed in series with the load, so there is no difference between an AC or DC circuit.
The only issue is you need an ammeter designed to work with the power source. For DCC you need something designed and calibrated to read accurately the current flow. It may be easier to just install an ammeter between the booster and the power supply, as the current will be proportional to the amount of current drawn by locomotives.
Below is a link about DCC amp meters. I built two ZXCT systems a few years ago and it is accurate. Agrees with the current reading in my NCE Power Cab which measures the DCC curent.
In the club NCE 5 amp Power Pro, it tripped the booster at about 4.97 amps. I have a high wattage rheosat for a load. Works very well witrh a Harbor Freight digital multimeter on the 20ma DC current scale which the author use to mention. This guy makes sure all his circuits work befor posting.
By the way, the HF meters measure about 13.6 VAC on the NCE systems.
http://home.cogeco.ca/~rpaisley4/DCCvolts.html
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.