Prevent refresh on back to group select

If train data is already known, don't refresh when going back to the group
selection screen.

view-train.js

@@ -343,7 +343,12 @@ function rsk() {
 
 window.addEventListener('popstate', function (e) {
 	groupIndex = null
+	if (trainData) {
+		onTrainData(trainData)
+	}
+	else {
 		refresh()
+	}
 })
 
 window.addEventListener('load', function (e) {