Hi, converted 2 BR 52 tender locomotives and I noticed the tub tender has a peculiar way of dismantling them,
a.) you have to unclip both bogies insert (1 has the slider attached to it)
b.) take 2 screws off (1 on each bogie)
c.) take the top of the tender off.
d.) a draw bar from the loco is also connected to the bogie.

by adding led's and changing the wiring within the tender its ok to take it apart but when you try to re assembly the tender top with the under frame, a some what complicated process takes place with other words its not the easiest process to do so, usually before the bogies had an independent screw which screwed into the under frame and 1 screw to secure the upper and under frame., further more everything is plastic, the bogies (used to have metal inserts for the wheels).
nowadays it may be done all by computer design and finding the best way where to put things but from a practical point of view its going back into a bad design period.
I'm sure there is a better way of designing such tenders.

another loco is the BR 18: the front light reflectors (design from the 70's) are more or less ineffective and the step platform on either side can cause the side rods to jam,
if there would be an extra support for the step platform so they can't bend inwards and therefore the chance to obstruct the side rods a major incident wouldn't occur,
I've removed the front light reflector and replaced them with tiny smd leds (1.5m x 1mm).
why would you use a chunk of acrylic plastic with no effect instead of using led's directly from the lantern.

It appears to me that some old methods and designs are kept and there is no one at Märklin to say: lets update our design strategy and apply a state of the art innovation.
Somehow I get the feeling there are sudden bursts of innovations but doesn't involve the inner workings of a loco., unless its a new design most of these locos are patched up from the original design and there are plenty of them.

John