New Electronic Trains

Hi,  The simple answer is yes. But many locomotives will not be able to manage O-27 curves. Providing your curves are large enough for the locomotives/cars in question you should have no problems.

Yes.

Track is fine.

If you are using an older power supply could be an issue for several reasons.

1) You may want to upgrade to a external circuit breaker. The circuit breaker in the old transformers reacts too slow to protect the engine's electronics.

2) You will not be able to access all the cool features with an old transformer.

I will second Chris's reccomendation about an external circuit breaker. I ran my TMCC locomotive off a ZW before I knew of this potential problem, had a derailment, and fried the electronics.  Lots of ways to do it.  I have TMCC direct lockon on one line, fused Power Masters on other 2. I find the least little derailment blows the fuse on the Power Master or trips the TMCC locon.

I'm learning there may be better ways than what I am doing, and will hope that once again the "real pros" will chip in or post the link to past discussions.

As Nicole said, they will run if they can handle the curves.  Many of the most desireable steamers require 36, 42, 54, or in some cases 72 inch diameter curves -- they just will not fit on tighter curves.  Most diesels are a b it more forgiving of curves but many of those are restricted to 31 or 36 curves, and there are a few, like the big Centipedes, etc., that want curves that are wider still.

With almost no exceptions, the catalogs always list the minimum diamter curve requirement for the loco.  In some cases, locos will run on a tighter curve (I have a couple of "O-36" rated locos that will run on 0-31, etc.) but it would be best to always take the manufacturers at their word and not assume any loco will run on a tighter curve than rated by its maker.

As Chris and  B&O Bill wrote, there are many choices for setting up external circuit breakers. Lionel recommended this for their larger transformers even in the days before all the electronics.
My favorite solution is to put Postwar Lionel #91 adjustable electromagnetic circuit breakers on each of the "Hot" terminals of my ZW transformers.

The built-in breaker is connected to the "Common" (U) terminals. It is possible to accidentally create a circuit between two "Hot" terminals (A,B,C,D) which would not be protected by the internal breaker. This can ruin a transformer (I've seen it).

If anyone decides to use these breakers be aware that if the adjusting knob is screwed down too far, the breaker will never trip.

I am certain others will suggest other, more readily available alternatives.

I see many folks recommend the addition of surge protectors too. Seems like a very good idea to me. Someone else would be in a better position to comment on those.

Regarding your track:

Some of the larger engines being made these days require wide radius curves, wider than traditional "O" gauge.
IMHO, all trains run better on nice, clean track. If your older track is in good shape, there is no reason not to use it. But if it is rusty, pitted, or has the plating worn off, you would probably be better off with new track.
Because of their expense, I do clean up remote control switches and uncoupling tracks. I do not bother with regular straights and curves.

Unless you're getting a big expensive train, most things will run on O-36 at the worst, and usually on smaller.  There are things that won't but they're generally not low priced or starter set locomotives.

Also, when manufacturers list what sized track a train will run on, they tend to rate it for track they make themselves.  For instance, MTH RealTrax comes in O-31, O-42, and O-54. If they rate a train, they're not going to say it runs on O-48 track even if it does because they don't make an O-48.  So they would round it up to O-54.  Likewise, their O-42's might run on O-26 and their O-31's might run on O-27.  If you have a specific train model in mind you can ask and someone is likely to know.