Some paksets (new Pak128 for one) could/will choose not to provide full slope for certain way types (or for certain ways, anyway).
For old savegame conversion purpose, if would be useful if missing images would be fetched in the nearest speed way/bridge providing it.
Example 1
Pak128 roads will have both heights, excepting expressways (i.e. roads faster than 90km/h).
Savegames with a 130km/h expressway climbing a full slope would use 90km/h images limited to those slopes.
Example 2
No Pak128 tracks will have full height support.
A hidden 80km/h railtrack could provide full slope support for older savegames.
This should work with both missing (Front)ImageUp2