Derailment out of portals.

I know this was raised some time ago, probably in an older starter pack. (I am using build 117009) At that time various suggestions were made and the problem seemed to disappear. However just recently I am getting a number of derailments from portals again. It seems to be restricted to certain types of locos. I have found the following to be a problem :-
Southern class 455/8 DTSO MSO TSO rakes <Kuid:45317:102523> etc
SWT Class 455/8 DTSO MSO TSO rakes <kuid:45317:100311> etc
Class 377/1 or 4 DMSO MSO PTSO rakes <kuid2:393563:100549:1> etc

No doubt there are a few more as well.

I seem to remember that one of the solutions was to alter the engine spec in the config file, but I would need guidance on that.

Any help would be appreciated.

Tony Howard