You can only enter the interior view (1) if the rolling stock has been built with an interior view. If you're hitting 1 and it's not doing anything it's that the caboose you're using doesn't have an interior view.
After pressing the "1" key, try pressing either of the square bracket keys to rotate round the interior cameras. Sometimes (certainly not always) that will move you into a passenger compartment.
Most Trainz items are built by the community. Each builder has different views about what they will include with their models. The upside is we have a very wide range of free items and we push the limits. The downside is not every item has the same set of features.