If the points are double points, with 1 point one way and one the other than yes they an derail if it is too tight. I have a large 66 fleet with over 10 in! I had a problem with my model of Lady Penelope Bachmann class 57/3 as it didn't like points AT ALL!! In the end I looked on the bottom were the wheels are and discovered a thin plastic detailing part( fitted to most models ) I removed them and vualla it was fine, I have never had a problem with 66s going over points. Another thing, do you have a coupler at each end because that is likely to be why as well because I had problems with some of my loco fleet as it had 2 couplers, I removed the additional coupler and it was fine. I hope this helps,
Thanks,
Swifty