go to cmp and find your loco, then right click find edit, then Content Creator Plus. then when ccp opens when your still on the main ccp screen look to the left of the screen and near the top it says traincar, right click that and scroll down to find interior then click it. then scroll to the bottom of the main ccp screen and find where the word interior is then click the ... botton and kuid selector will open. then find a nice interior and click select.
then click ctrl s on your keyboard to save it then exit it and then commit the assests on cmp then open trs2006 and u have a interior in you loco.