Digital this time.

I've discovered "tram lines" (well, a single tram line) about one pixel
wide running vertically through a series of images I've taken at night
using long exposures. No sign of the same effect in ordinary pictures
taken in ordinary conditions. Unfortunately I'm currently about 8,000
miles away from where I've stored the other night-time pictures taken
with the same camera but I don't recall seeing tram lines previously.

I'm wondering if this is a known effect with low light levels and CCD
devices or something else is happening?

I take a lot of photos using roll film so I'm used to tackling tram
lines in that context but in digital? What's going on?

Camera is an old Fujifilm S1500 with a 10Mp sensor, if that's relevant.

Thanks,

Nick
PS, please don't infer anything from any lack of response from me at any
time. We've had several bouts of periods of days without electricity out
here and no electricity = no internet so connecting to uk.d-i-y can be
an issue somethimes.