When I was last there - pre-Covid - it was the 4-car 464s. Loadings are generally good throughout the day - remove some more single line (but where?) and it would justify 4 trains an hour.
I cannot find it now, but I previously looked at the route, and there are some possibilities to double track the line. In short:
Malaga Centro-María Zambrano: this is a bottleneck, but if the railway could be extended under the Guadalmedina into the centre of Malaga (to the downtown port?), it would be worthwhile.
Airport (roundabout)-Los Alamos (before)/after Plaza Mayor: this could be double tracked with minimal fuss. I don't know why it wasn't done as part of the other airport works.
La Colina-east of Torremolinos (tunnel): this could be done. There's (just) enough space to fit a second track through there, including building a second short tunnel.
Torremolinos tunnel - worthwhile and possible. It would allow double track from Torremolinos to Malaga, which opens possibilities of running short services to/from Malaga from Torremolinos.
West of Torremolinos - there's a section that would be impossible to upgrade, but it's only around 300-400m.
West of Torremolinos (after the impossible-to-upgrade section) to east of El Pinilo: easily upgraded.
Benalmadena-Arroyo de la Miel - east of Torremulle: this section is tricky. It's in a deep cutting for much of the length, and I don't see easy options here.
East of Torrequebrada - Torrequebrada tunnel: very difficult to upgrade.
Torrequebrada tunnel-Los Boliches: I don't see why this couldn't be done. It should be possible to put two tracks through Torrequebrada station (just) with some redevelopment of the area, which would allow a twin track tunnel.
Los Boliches viaduct: GET RID OF IT. Put it in a tunnel where it belongs, as the viaduct is an eyesore and is a huge barrier.
Fuengirola tunnel: shouldn't be much of a problem to extend the existing cut and cover tunnel.
So there's really only a short section of track that can't be upgraded. While some of it may not be worthwhile, Fuengirola could really do with the removal of the viaduct and the extension of the tunnel.
The major problem is that no-one wants to make a decision on the future coastal line. Everyone agrees that it's needed, but the speed of the existing line leaves a lot to be desired.
The existing line cannot cope with any extensions, but I think the best option would be to leave the existing commuter line alone (with some upgrades, such as the removal of the Los Boliches viaduct and the upgrading of the Plaza Mayor-Airport section) while building a new line capable of accommodating the AVE from the airport towards Marbella, Estepona and Algeciras. You could build an existing stop somewhere around Mijas to serve Fuengirola/Mijas, a stop in Marbella, one in Estepona and terminate in Algeciras.
