So I think its pretty sure that this feature will be officially activated in the next add-on pack.
I currently have no clue as to why they are not activated in the current game,
besides of course that the graphics for them are not finished (hint hint),

except for one station (TRSTAT1.DAT) (TRSTAT4+5 use the same images as TRSTAT1)
But there might be other reasons for this.
for those that like to play around with them.
To activate, add a field[4] and set it to 1 (bitmask field)
* Diagonal station don't show cargo icons on them. (might be a other setting)
* savegame loading is not effected by field[4]
* platforms of nearby station overlap
Have fun.