hello, what is the best way to get to koln to wengen by train ?
what trains to take and from which station to change

http://www.bahn.de/i/view/DEU/en/index.shtml

Better:
http://reiseauskunft.bahn.de/bin/query2.exe/en?ld=9642&seqnr=1&ident=1b.02086242.1377774190&rt =1&rememberSortType=minDeparture&HWAI=JS!ajax=yes! &HWAI=JS!ajax=yes!&HWAI=CONNECTION$C0-0!id=C0-0!HwaiConId=C0-0!HwaiDetailStatus=details!;~CONNECTION$C0-0!HwaiMapStatus=UNDEFINED!HwaiMapNumber=UNDEFINED! HwaiMapSlider=UNDEFINED!HwaiDetailHimMessage=UNDEF INED!;

Adjust the departure date and time to suit.

Using the above site (first one) enter 'Koeln Hbf' for Cologne's main train station and 'Wengen (CH)' for Switzerland. Enter your proposed travel date and various options for the day will come up....

All trips are between 7 and nine hours, with 3 to 4 changes.
The (faster) 7 hour route takes changes EITHER in:

Basel/Interlaken Ost/Lauterbrunnen
OR
Mannheim/Interlaken Ost/Lauterbrunnen.
You choose which time of day you'd like to travel and purchase ticket.

If traveling much in Switzerland investigate a Swiss Pass or Swiss Card or Half-Fare card - in which case you would only want to buy a ticket from Cologne to Basel or other border station and then use the pass. It doesn't take much travel in Switzerland to make such passes pay off - they are valid in full to Wengen and once there cover in full many conveyances or give 50% off things like aerial gondolas, etc.

For lots of goodies on Swiss trains check out these IMO fantastic sites - http://www.budgeteuropetravel.com/id3.html; www.swisstravelsystem.com and www.ricksteves.com.

Thanks mokka, I didn't realize that the search wasn't stored for long.

the only tricky part for some at least with changing trains is at Interlaken-Ost, jumping off point for trains to Lauterbrunne, Grindelwald and Wengen - beware that this train splits at Zweilutschene - with half of it trundling up to Grindelwald and the other half to Lauterbrunnen - so be sure you are in the correct part of the train - I take this train a lot and there are always flummoxed passengers told by the conductor they are in the wrong half when checking tickets and then they have to scramble out and board the other half of the train at Zweilutschenen - and as these trains are often full IME they may then have to stand the rest of the way. Each trains car is clearly labeled as to which of the two terminal destinations they are going so look at plaques on the outside of the car before clambering into it.

Keep in mind that Interlaken has two train stations - West, the main one right in the heart of town and just a mile or so to the east East (Ost) train station where all trains from the west terminate (have to due to differences in track gauges - standard gauge but trains to the east (Brienz, Brunig Pass, Lucerne) are only narrow-gauge - so Interlaken-Ost is the end of the main line and all trains will terminate there if coming from Basel or Zurich - Ost is of course the jumping off point for trains into the Jungfrau Region - Grindelwald, Lauterbrunnen, Wengen, etc.