Have items sent to the home of a friend, or the office. This way your wife won't intercept delivery of trains. After all, the return address of the vendor is usually displayed, and many ship in boxes gotten from manufacturers. I took it down the hobby shop last week to get the E unit recalibrated, dear. If you're a steam fan, it is easier to conceal new trains.
Most steam engines are black and they blend in.
Since most wives are not experts in spotting the difference between a pacific or Hudson, you can get away with it. Diesels are another problem. Though the wife might not know an FA2 from an E7A, she will get to recognize colors and maybe even a few road names. Be careful when you bring out a diesel with a notable paint scheme - especially one that isn't already in your consist. Conspicuous schemes include EMD demonstrator SDs, BNSF and other new roads.