i have an athearn sd40-2, but the front axle keeps on derailing at 18" curves. my guess is that my tracks are a little misshaped already (and that because i do not nail my tracks on my layout; i want the layout to be still modifiable since it is still pretty simple). are 18" curves the minimum for my 6 axle sd40-2? my 4 axles do not have a problem. oh btw, just to confess, my other 4 axles include 2 F7As and an athearn rdc. so i was preferring 1960 era engines. but my dad got me this sd40-2, and i'm not strict on the eras. i would love to see this sd40-2 running along the track.