Engineering units are Infantry units that display the "Engineer" icon in the bottom of the top-left section of their "Technical Readout" information pop-up window.
The presense of one or more Engineer units in or near a hex owned by you will accelerate the rate of construction of any road, railroad, plant, base, mine or other improvement being constructed inside that hex.
Although engineering units possess a "Construction" order on their Orders menu, it is not necessary to explicitly order them to "Construct", since the game will automatically detect their proximity to any construction project.
Additional Engineers will also improve the rate of construction and completion of the project.
Engineers cannot speed production or repair other units.
The presense of an engineering unit will enable the construction of upgrades within hexes that lack supply.