Interesting that not many here leave things out overnight.
We've been leaving pretty much everything outside continuously from the yearly early summer opening to late summer closing of the layout. The only time we bring anything back up to the house is if something needs servicing, or a battery charge. At summers end everything comes back inside for winter storage.
Are all of the responses here just for locomotives? Or do you really bring in everything at night?
We typically keep around 5 locos on the layout. Usually 2 LGB moguls, an Accucraft #5 goose, an Accucraft Whitcomb, and an LGB 2063 Davenport.
The 2 moguls are protected on twin 6 foot bridges which get wrapped in a heavy plastic tarp when not in use. The Goose and Davenport are protected in a roundhouse. The little Whitcomb goes into the mine for protection.
Here are the bridges with the plastic tarp (note the tarp is not secured nor wrapped in the photo), and mine:
And here's a link to a too big for MLS roundhouse photo:
Roundhouse
We also use plastic storage bins to cover structures and some rolling stock we don't want getting too dirty from potential rain splashing.
But this is only during the summer when it's relatively dry in Southwest Colorado. Yes the late summer monsoons pose a problem, but our covering system has worked very well thus far. If we tried to leave structures and rolling stock out in the winter, the snow would likely crush them.
It also helps being out in the middle of nowhere sometimes. Not too many "punks" as Mik mentioned around here. About the worse thing we have that might "take" something away from the layout are
pack rats. And I think we're safe from them taking any locos since the locos are bigger then they are