In the meantime I added a little power amplifier with an LMD18200 (Bridge), and wrote some lines of code for a very simple software (and easy to understand, I hope) to realize a "one loco" command station.
here is the additional hardware (electrolyte capacitor at pin6: 10 to 100uF, C1, C2: ceramic capacitors - please check that your arduino accepts your input voltage of 12...15V, or use USB power supply for the arduino!)
and her the software (limitations: only 1 loco, only speed and direction controlled - "switching the function off and on is left as an exercise for the reader ...").
To generate the DCC signal timer2 is used and a simple state machine (1. preamble, 2. separation pulse, 3. send a byte) - a special thank here to Wolfgang Kufer and the "openDCC" programs , which are not explicitely used here, but which were very helpful in understanding DCC.
For the "advanced readers": There is a general Arduino DCC library available at model railroading with arduino - this can be used for more useful programs. However, the source code of the library is pretty complex.