Arakkonam (formerly Arkonam) is well known to railway followers because it is an important junction as well as electric loco shed, but has recently come into prominence because the inundation of Chennai airport caused some commercial flights to be operated from there. To be precise, this is the NAS (Naval Air Station) at Arakkonam which the Navy calls INS Rajali.
Most basic information can be seen here:
Although it started off as an IAF base in the 1940s, it was abandoned soon after WW2 and was reactivated for the long-range reconnaissance aircraft of the Navy during the late 1980s. The TU-142s and now the Poseidon P-8s have made good use of the 4.1 km runway which has been claimed to be the longest military runway in Asia.
Here you can see the locations of Chennai international airport (MAA), IAF Tambaram and INS Rajali marked with the small gold stars.
One can see that INS Rajali is about 50 km west of MAA, while IAF Tambaram is only 10 km away. At least there is no chance of a confused airline pilot landing his 747 at INS Rajali by mistake, though this has happened once at Tambaram in recent years.
Here is a closer view of INS Rajali:
Though it is not very clearly shown, the railway line from Chengalpattu runs along the highway right by the boundary wall of the base. The Railways have been planning to electrify this section for a long time but the Navy have objected to the presence of the traction equipment being an obstacle to the flight path. Thus an alternative line is being built further from the airfield, but this seems to have dragged on for several years.
The diverted rail line was completed in mid-2019. EMU trains could now run all the way from Chengalpattu to Arakkonam. It was proposed to run circular EMU services on the Chennai-Chengalpattu-Arakkonam-Chennai in clockwise and anti-clockwise directions.
This new line is not shown in this map.
This is not the first time that military airfields have been used a a backup. Sulur for Coimbatore and Avantipur for Srinagar are other examples. The inaugural flight of Jet Airways to Coimbatore did land at Sulur by mistake. Apart from the Saudia 747 which wrongly landed at Tambaram, there have been several incidents including a mid-air collision and another which totalled a JAL DC-8 which were caused by the proximity of BOM to Juhu. More about these later.
With all these movements of heavy aircraft, it is fortunate that this airfield has not seen a major aviation accident yet. However, India’s experimental AWACS on an Avro frame did crash a few km away in 1999, apparently putting an end to DRDO’s efforts in that direction.