If you don't mind micro-managing the construction units, here is something I do every turn with my construction units to make sure they're working on what I want them to work on:
1. Pull up the CR report and filter to only show Construction units.
2. Click on "All" so it first shows on-map units. If I see any units other than the railroad repair units, I know they're available to reassign to a base, because they've already done the repairs on the RR in the hex they're in.
3. Click so it displays "Support" units, then sort by location by clicking on the header. All the construction units manually assigned to cities and towns (and airports) will appear at the top of the list.
4. Click on their hex coordinates one-by-one to see status of repairs, including if the airport building has gotten to the next level, in which case you need to manually "Build Air Base" to start the next phase of construction.
5. Once I have my list of on-map construction units and units assigned to locations that have finished their work, I can manually assign them to the next city or town that needs port or railyard repairs, or an airbase expanded.
BTW, my personal experience is that assigning more than one construction unit to a city doesn't increase repairs or air base construction. Does anyone have a different experience?
Nevertheless, sometimes I assign multiple spare construction units to a location near where I know I'll need them in a turn or two, just so the AI doesn't assign them somewhere else. This can become a problem when those SHAEF construction units come in as reinforcements and the AI assigns them to repair the RR in a hex in Sardinia that I don't care about. If you click "Return to HQ" it send them back to W. London, but you can't assign them to locations in Italy them from there. If you change their HHQ to 15th Army Group or 5th Army, then you can't "Return to HQ" that turn. So I manually send them to a town in the theater I'll need them just so the AI keeps its hands of them.
Does that all make sense?
Cary
