Comments on Modelltrack piece 3900ZD at either 56mm (=calculated from geometry) or 57mm (=measured on several track pieces) as discussed in maerklin-user.net see: http://www.marklin-users.net/forum/yaf_postsm428640_Model-Tracks-3900-and-3800---Modellgleise-3900-und-3800.aspx#post428640

Currently in the file "mrklnholde_v1_4.xtp" the 3900ZD is still at 56mm by following definition in the xtp file:

TURNOUT HO " Maerklin Modellgleis      56mm     3900ZD"
    U "Gerade" "jr" " Maerklin Modellgleis " " 56mm " "3900ZD" 2.204724 1.476378 0.053333 16754688
    P "Normal" 1
    E 0.000000 0.000000 270.000000
    E 2.204724 0.000000 90.000000
    S 0 0.000000 0.000000 0.000000 2.204724 0.000000
    L3 16754688 0.053333 -0.000000 -0.787402 0 2.204724 -0.787402 0
    L3 16754688 0.053333 0.000000 0.787402 0 2.204724 0.787402 0
    END
# Note, the German Maerklin catalogue of 1955, page 34, erronously specifies the sectional track 3900ZD with a length of 58 mm.
# However, the English catalogue on Modeltracks specifies it with 2 3/16 inch, the same number and length as the sectional track 3900 D 1/4.
# The geometry combining two turnouts (16.875 degree, radius 585 mm) with one crossing (177 mm) requires
# compensation by sectional tracks 3900ZD (56mm) and 3900DE (115 mm) for the length of one turnout attached to the crossing.
# From this geometry, the sectional track 3900ZD is confirmed to have the length of 56 mm! as correctly indicated in the English catalogue.

You can easily increase the length by exchanging values as follows using a simple text editor on the file "mrklnholde_v1_4.xtp":

"2.204724"  values replaced by "2.244094" and
"56mm" replaced by "57mm"