Things aren't always black and white (even if they are black and white). Do not fear, the driver hasn't abandoned the train – I was in fact in the last car, facing backwards. I was rather pleased with the resultant image, as I managed to avoid significant blur* from the slow 0.3 second shutter speed (even more so when considering that the lens doesn't have optical stabilisation), handheld.
Around 1/125th of a second is often quoted as the minimum shutter speed to avoid shutter-slap issues from the A7r's somewhat hefty mechanical shutter.
The weather's been pretty miserable in the last couple of days – overcast and dreary, with occasional light rain – and it doesn't look like conditions will improve until later next week. I'll still probably take a photo or two, at any rate!
*Having said that, at 100% view, there is some blur on the pixel-level, but when resized to web-suitable sizes, the blur is negligible.