If you want to have a quick look at how the feed looks like, there is a web client for mqtt at http://www.hivemq.com/demos/websocket-client/. Don't leave it open for too long though, it might kill your browser (Chrome struggled a bit on my old laptop).

    {"VP":{"desi":"N","dir":"2","oper":"XXX","veh":"H9449","tst":"2017-05-25T11:31:50.000Z","tsi":1495711910,"spd":2.22,"hdg":10,"lat":60.249649,"long":25.008106,"dl":0,"oday":"2017-05-25","jrn":"XXX","line":"XXX","start":"1439","stop_index":7,"source":"sm5logger"}}

HSL provides the bus, tram, suburban rail, subway and ferry system in the Helsinki metropolitan area.

I've written a few toys that use their GraphQL API to show real-time displays of trams. Pretty neat system.

Thanks! Waiting for your MQTT integration ;-)