are you using TRS2006? i too was having this same problem with CMP in TRS2006. Here are some of the steps i did based on feedback from others in the forum.
Please note this is for TRS 2006.
* On launching the CMP, if you see this message and you have been waiting 10-15 minutes, press the ESC key to bypass the updates.
* If this does not work, go into Task Manager, Process tab and kill the Launcher.exe and ContentManager.exe. and then try again.
* Now, if this does not work, you need to do something drastic. Go into C:\Program Files\Auran\TRS2006 and delete the assets.tdx file. Sometimes this get corrupted but dont worry, Trainz will rebuild it.
* In my case, even this did not help so i did the final step, go to the cache folder (C:\Program Files\Auran\TRS2006\Cache) and delete all files/folders in there.