MTH locomotives and.....

First, a BIG THANKYOU for all your replies to my past questions.  You all have been very helpful.  I am new to G scale and have taken your advice. So........
1.  In your replies to my past posts you mentioned that USA Turnouts are really good.  Well, I got some of them and I am very impressed!  The #6 is a beautiful turnout!  Question: what diameter of rail do I need to parallel the tracks?  I assume a 20 diameter. Correct?
2.  More importantly, I am interested in purchasing a locomotive from the manufacturer of MTH.  In particular the  
    4-8-4 Gs-4 Steam Engine with Proto-Sound 2.0 - SP (Daylight)Cab No. 4449.
    It is a beautiful locomotive but what is the quality like and detail?  They have a beautiful Challenger too.  Anyone out there who has one of these or knows anything about MTH?   One thing that concerns me is that their track has a center rail.  What's up with that?

Anyway, thanks for your responses! 

UncleHans