I looked at a number of texts to find out what this “cajón” is, and it’s definitely not a caisson. The rail traffic goes through it, and it is built with diaphragm walls (muros pantalla).

Otro aspecto relevante es el del vaciado de la zona para instalar el cajón ferroviario de hormigón por cuyo interior circularán los trenes.
http://www.diarioinformacion.com/secciones/noticia.jsp?pRef=...

..la construcción de los muros pantalla que conformarán el futuro cajón ferroviario avanzan con un mes de adelanto sobre los plazos establecidos…
http://blogs.larioja.com/nanay/2010/4/21/la-construccion-los...

Tal y como explica el Ayuntamiento de Logroño y parecen saber todos y cada uno de los vecinos de la zona preguntados por Diario LA RIOJA, la redefinición del citado mapa de accesos obedece a que sólo así se podrá completar la construcción de los 'muros pantalla' del cajón ferroviario necesarios para 'soterrar' las vías del tren.
http://www.lif2002.com/es/not100819txt.htm

I found these similar texts in English, and they refer to a “box”, and more specifically a diaphragm wall box:

Largest ever UK diaphragm wall station box…The main element of the package was a 65,000m²
in situ reinforced concrete diaphragm wall forming the retaining walls for the underground
box housing the new Station….
http://www.skanska.co.uk/global/Sevices/Piling/Datasheets/St...

The trains leave the splendidly refurbished Barlow Shed at St Pancras before entering the tunnels at London West portal on their way to Stratford International station situated within the 1 km long diaphragm wall box before emerging at Ripple Lane east of Barking.
http://subs.geplus.co.uk/Assets/Documents/Supplement40Years....

The platforms for Stratford Station are contained within a 1km long, 35m-52m wide, 12m-25m deep underground box constructed using diaphragm walls and an in situ concrete base slab. The box accommodates two 425mlong international, two 290m-long domestic platforms, and associated up and down lines. There are also up and down fast lines for through trains and a rail connection
from the middle of the box to the surface to a future depot at Temple Mills. The box is long to accommodate the platform lengths and the fast 160km/hr turnouts that enable trains to transfer from the main through lines onto the platforms, and into the tunnels. The station slab that supports the station spanning across the box at ground level is carried on the box walls and intermediate columns
supported on plunge columns and barrettes.
http://www.arup.com/_assets/_download/download234.pdf

The Mater Stop Contractor provides a diaphragm wall box structure with unreinforced
concrete blind panels outside the reinforced box panels at both ends of the Stop.
http://www.rpa.ie/Documents/Metro North/Metro North RO Oral ...

I’m quite convinced “box” is what you need for the cajón part. The rest of your sentence states that it's a rail infrastructure, so I think diaphragm wall box would be just fine.