wierd vertical lines

Have a look at this odd structure of vertical lines. Very regular with a perfectly horizontal cut-off. I was trying to increase the cloud structure by duplicating the sky and steam, then using "multiply" as the blending mode. Normally this works well, but for some reason I've ended up with this.

Its in Photoshop v7. Anyone understand whats happening?

Richard, maybe if you upload the original so that others could play with it?

Tony,

Sadly I dumped this attempt and have since reworked the shot from scratch without this weirdness recurring. Its the first time I've seen anything like this.

Hmm, well, OK then! It's hard to figure "weirdness" sometimes!

Heres a link to the better version:

http://rcb4344.zenfoli​o.com/p538568564/e471b​e8a4

Cool! I'd say the UK has nice trains, and you got a good shot!