Here is a update for you all, on my issue. I have the gauge now. All wheels and flanges check  out to be spot on and no issues.   
I found  one of the span bolsters to appear to me,  to have  excessive movement / or  play,  when compared to the other one on the frame.    I removed some of this  'play',  and now the car rolls down the track and over every crossing with no issues. ! hurrah!  problem is solved.
My thanks to each of you for your ideas and guidance!...they each educated me, and guided me to understanding better,  HO rolling stock, track, and to look closely at the wheels and span bolster and how important it's function is on this toy.   

LD limit   reads 125000

But is that  not just  there for decoration?


My thanks to you both for your very helpful  information and great assistance!

To answer:
yes everything moves and pivots and rocks freely, nothing  hangs up.
Car derails at several spots, and all spots are at random. with the exception to that being  the  60 deg. crossing.where it always de rails.
Have NOT checked wheels to be in gauge.  I Will do when I purchase the said  gauge checker. I had no prior knowledge of this tool's existence.
Am i going to use this gauge to check the distance BETWEEN wheels or the ind. wheel height?
Is there a certain model No# gauge I should shop for  to check  the 33" wheels?  or is there just one gauge for HO scale trains?

Much appreciative of your sharing the weight rule, that will be very useful.  

I have a Bachamn  made, N&W  black hopper car, with numbers : N&W12988 .
It has Kdee couplers.
it needs some axles, I am thinking of using metal axles and wheel type,.
But I don't know if it  needs  33"  or 36" and can't find  info to tell me for sure
I do not have the original box, I bought it used.

Can any one help by telling me which?

Hi, I have a used  bachman  heavy duty  / heavy load hauling flat car.
it reads on the side: Louisville and Nashville 24562
it has 4 trucks. and a red plastic frame.
The present wheels and axles are plastic.
It rolls pretty well except that both trucks closest to the middle of the car, frequently derail when  crossing a switch or crossing, or it the car is placed any where in the train other than  very close to the end of the train.  and I want to correct that.
I have replaced the axles and wheels of the inside trucks with new plastic types but still little improvement.
I also glued metal BBs to the underside for added weight, which I think helped some as the derailments are not as frequent after doing so......,
and I don't think I should add still more weight?...as the car now weighs 4 oz.
can i safely add more weight?..but how much more?
So I am thinking of installing metal wheels and axles on the two inside trucks,  in hopes that will help or correct the problem,
but I don't know and can't find  info to tell me for sure if  this car needs 33" or 36" wheels, or just what size.?
Can any one help by telling me which?
and I welcome your suggestions and cures for the derailing issue.