First take a screen shot,get it out of your user data in the trainz folder. open in a paint program,and resize it. Send to desk top.Make a note of screen shot name.Now go into CMP right click on the asset you want to add the thumbnail to go down to edit,edit in explorer,when window open drag your screen shot in the list. Now the Config file double click on it open to max.if you have the information of thumbnail then just add the name of your screen shot.