I’ve often thought about installing an on off switch on sidings etc but still have not done so. I leave legacy and TMCC engines sitting on a hot track while running other engines and have not had any issues. Been doing this for years. My thought (which doesn’t mean much) is that an inrush of 18 volts really smacks the electronics. After initial track power is turned on then all boards are accepting the 18 volts as they are designed to handle for long durations.
Anything you turn on, tv, espresso machine, air conditioner, light bulb, etc., anything and everything, gets power applied immediately. Do these things fail earlier than normal because of an inrush when turning on? Not likely, but ...
It comes down to this: Did the person who designed the input circuitry inside your locomotive, vs. your tv, espresso machine, or air conditioner, properly take this into consideration?
If you believe the answer to be yes, then don't worry about it.
If you know that the answer is no, don't use it, or even buy it.
If the answer could be no, then by all means don't turn it on and off unnecessarily.
Which do we have here?
Is Lionel stuff, especially the electronics, generally robust? Yes.
Are there occasionally, but fairly rarely, problems? Yes.
In light of this what's the definitive answer?
There isn't a clear one, but I'm leaning toward "Don't worry about it".
Why? While we know people come to this forum for help with occasional electronics issues, in general there are not many of these issues. Additionally, there are some well-known chronic problems, but also not many.
I know that there are several, perhaps many, people who will disagree, especially if they've experienced a failure themselves.
Think about this however: We're only hearing about the ones that have failed. What we really need to know also to put this in perspective, and in extreme cases whether there's a crisis, is how many good ones didn't fail?
And this we don't hear about in this forum -- ever.
Don't worry about it. For almost everything in our hobby there are far more good than bad, and for those things with chronic problems we have our friends here reporting them so that that we can avoid those specific offending products.
Mike