9V should give you a relatively sedate speed. I assume you wouldn't be hooking these up in series, which would yield 18v and would indeed make for fast movement. 12-14v generally yields an acceptible top speed in most LGB locos. I think LGB use 22-24v motors for two reasons: More volts help push the electrons through dirty track/wheel, and little kids (and some big kids) like to run their trains fast.
You could verify the speed at 9v before ordering from Conrad by simply buying a 9v alkaline battery at your local Auchan/Leclerc/Carrefour/etc. and connect it to see how fast it runs. This won't be exact, because the 9v rechargables will likely be either 8.4 or 9.6 volts, since NiMH and NiCd cells are nominally 1.2 v each, but it should be close. BTW, 250 mAh batteries won't give you much running time. I generally use 2000-3000 mAh.
If 9v still makes your trains run too fast, you can use a resistor (a big one rated for say 5 ohms and 5 watts) in series with the motor to step down the voltage. You can also use diodes in series (a la Carl Hibbs and his vibrator) or build one of the speed control circuits using a darlington transistor mentioned in the forum from time to time. As I recall, Carl has offered to build these for other folks before. You could also apply any of these with your 14v drill batteries.
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