Crossrail
There are only two airports that stand to gain significantly when Crossrail opens: Heathrow and Southend. SEN will gain as the eastern end of the line opens first. There will be easy step-free interchange with trains to and from Southend and when fully opened the line has direct access not just to the city and docklands, but the west-end as well. LCY will be exposed to increased competition from LHR,also interchange between the DLR and Crossrail will not be great, as they will be on different levels where connection is possible.
None of the above means a queue of airlines begging for a slot at SEN; but it's all good in the medium to long term. LTNman has pointed out that Southend is the only airport with two mentions on the Crossrail map. When printed hundreds of thousands of free maps will be picked up from stations (and airports) by all sorts of travelers in London and beyond.
To me, Crossrail will work in SEN's favour. It has to be remembered that ambitions are modest, currently just aiming for 2.5 million pax. So a 17 billion pound infrastructure project could reasonably bring gains to businesses with good connections to it; gains that would push the airport back in the right direction.