I am not sure but this is what comes to mind. Draw it as you go, so you can picture what I mean.
The row started with an E (for empty.) Then within the row there are 5 pairs of (Truck,Empty)
So that accounts for 11 spaces. there are 4 spaces left to fill
Some, or all could be on either side of the allotted pairs.
So there are 6 places they can go. (they can all be in the same place)
So there are 5 bars. You can draw these through the middle of the circled TE pairs.
The 4 spaces are represented by stars (stars and bars method)
So I think I need to choose 4 (or 5 which is the same ) from 9
9C5=126