am i correct

[Daylight4449]

the usra light mountain by spectrum can handle 18 inch radius right?

[Nigel]

yes - but it does not look good doing it, and the car behind it or the tender may derail, depending on the car behind it.

[Daylight4449]

hmm... i dont have any curves but many switches of various sizes... will need to think.

[ABC]

yes - but it does not look good doing it, and the car behind it or the tender may derail, depending on the car behind it.

I find that running it makes the tender derail almost all the time and when the tender doesn't derail the next car does just like Nigel said. So it isn't really feasible, but it runs fine on 22" radius.
I'd suggest #6 or #5 turnouts, I'm not sure about #4 turnouts. But standard turnouts will cause derailments and it is almost sure to derail if it is going in reverse.

[Daylight4449]

so 5 and 6 are fine? but not 4?

[ABC]

so 5 and 6 are fine? but not 4?

I couldn't tell you because I haven't run it on any number 4's (because I don't have any). The bigger the better when dealing with turnout and radius size. It is best to go with the largest radius or turnout that you can fit. #6's will work flawlessly as long as your turnout isn't defective, and #5's should work too, the only question may be while backing up, I haven't tried running it in reverse on #5s