yes, "au train où vont les choses" means " à la vitesse où vont les choses".
But I wouldn't say that it's idomatic, since you can find other examples. Train is an rather old way to say speed, it's exactly the same meaning.
Here are some quite common expressions:
"rouler à un train d'enfer" = "rouler très vite"
"à un train de sénateur" = "à une vitesse de sénateur" = lentement
and also "le train-train quotidien", here the repetition of "train" gives a "boring" feeling