When the vendors say 072 or 054 etc. they have tested many complex turnouts and radius combinations and select the radius that does not present issues. In several cases on my simple layout, my engines run on a smaller radius than advertised. In one instance, the coupler wire pulled out and the coupler now does not open; however, it does not effect my layout operation.
This is true for all the major vendors. They rate an engine at the smallest radius they sell that it will run through a set of switches and S curves on.
This does not rule out a slightly smaller radius made by someone else that it will run on, but it gives you a point to go by.
It probably will not run on anything near the next tighter curve the engine vendor sells or they would have tweaked it to get that rating.
The only Sure way to know is to get someone that owns one to try it.
I expect getting someone to risk a New $2000 VL Big Boy is going to be a hard sell.
Bottom line Frank; Good Luck. O-60 is a lot tighter than O-72
