This afternoon I extended the tests. Some extra information:

The serial port in the rpi ttyAMA0 does not seem to be able to generate the dcc signals. The tests I made where with a Motorola loco.

TAMS (motorola) OK

Loksound (dcc) Not OK

DIY acc decoder Not OK

The work around so far is to make use of a USB-Serial adaptor:

TAMS (motorola) OK

Loksound (dcc) OK

DIY acc decoder OK

I will post if I found out more…at this point it is clear considering the fifures above that the ttyAMA0 is no generating the entire signal. It seems the ttyAMA0 is not capable of changing the baudrate.