That definitely looks like a water guage! You had to watch those things like a hawk and make sure you had enough water for steam to get to your next rest stop for water and fuel, and sand, if you had to go up grades. You also had to make sure you had enough water to cool the crownplate, 'cause if that went you'd have an incredible explosion that could take out half a city block!! Smart railroads always kept an older experienced locomotive engineer with a trainee, until he could be trusted wirh good judgement with operating the equipment over that route.  Lives are at stake at all times while operating a steam locomotive, even the Bon Flying Scott. Diesel locos will never seem so alive, or be as dangerous