Motion blur is a funny thing. Too much--which is what happened on these images--and your subject disappears in a mist. Not enough and it just looks like unintentional blur. In either case you lose the effect you're going for.
When you're locked in to an ISO and aperture number, your only choice to adjust shutter speed (further than a stop, anyway) is to adjust in development. These images below suffer from far too much motion blur, so shooting Tri-x at 1600 instead of 400 ISO would make a big difference. When accounting for reciprocity failure, An 18 minute exposure theoretically turns into two minutes.
I say 'theoretically' because of the incredibly wide latitude that is inherent in Tri-x. Even at more conventional shutter speeds, underexposing by a stop doesn't result in significantly less exposure on the final negative. So, all things equal, that original exposure of 18 minutes could be as little as five with no adjustment in development time.
Or heck, throw in another 30 seconds in the tank just because you can. There are lotsa variables here when it comes to long exposures and film. My takeaway is don't be afraid to close that shutter after you've captured what you need to.
|18 minutes. Too, too much motion blur.|
|Picking up trains|
|Four year old playing with trains|
|Backyard under moonlight. Funny, I don't remember shooting this.|
|Getting the spooks in the house to do a few dishes.|
|Getting ready in the morning|