There is also a sharpe bend after Balham,might want to keep an eye on that aswell.
There are a few faults i noticed when i had the London- Brighton add on so i took it back to shop and swapped it for Cambrian route. I waiting a few months and got the London South Coast route which is much better. It has the same London - Brighton part on and also the option of going via Redhill. It also has other routes on and i would say just buy that one. You don't have to wait a few months its out now.
Also if you carn't find add ons for train sims in any store and don't want to buy over internet like me, go in HMV and they can order it in for you, always very helpfull HMV.