[…] mais au train où allaient les choses […]
I couldn't find the different pieces of this as expressions or idioms or anything. I can't seem to parse any meaning out of it.
To me, just saying this way doesn't give the temporal meaning of the idiom.
To understand it fully, one must see that this idiom is related to the meaning of the word train which is (in french) synonym of allure, rythme, cadence, vitesse (see III. in the CNRTL definition).
There are many idioms with this very meaning of pace, speed or maybe rate, like aller bon train (= to go quite fast), or aller à un train d'enfer (= to go extremely fast).
...could be translated to
But the nature of the pace (fast or slow) is implicitly deducted from the context. It could just also be :
... meaning ...
It means if things keep going on this way, or according to larousse.com
And to the TLF
It may¹ convey a meaning of pace, too. As in if things keep on at this pace, but I think that it is not that important, for instance both could be glosed si ça continue comme ça.
Even if according to someone from my dear LIMSI it should be du train dont vont les choses
1. Merci, Romain.
See other answers for more detail, but the quick answer is that the English equivalent is "At this rate...", or "At the rate things are/were going..."
D'après le Robert, parmi les différents sens et emplois de train, pour ce qui concerne le déroulement d'une action :