AvG,
I have moved this post to the Pak128.Britain board, as this issue does not seem to be specific to Simutrans-Experimental, but rather is a Pak128.Britain issue. Can you post a screenshot to explain the problem that you are having with the vehicles? I must confess, I am not intimately familiar with the calibration of the length parameter (you should no longer mis-spell "length" as "lenght", as that has all been corrected in the source code in both Standard and Experimental) in paksets; when I produce vehicles for Pak128.Britain, I just adapt existing vehicles.
I don't know whether this is relevant to your particular problem or not, but there has recently been a change to the Simutrans graphics engine to reduce graphics errors: see
here for details. It is currently available in the latest nightly builds of Simutrans-Standard, and should be incorporated in the next release (7.3) of Simutrans-Experimental.