interlock

To fit or clasp together securely.

To interlace.

A safety device that prevents activation in unsafe conditions.

The demonstrator couldn't figure out why the machine wouldn't work, until he remembered that there was an interlock so it wouldn't operate with the cover open.

Features such as trackless doors, mercury-type door interlocks, roof-mounted door fault indicator lights, rubber window glazing, improved retractable shoegear and a modified electro-pneumatic brake system designed to facilitate maintenance and improve reliability, which have proved their worth on the prototype trains, are continued in the new stock.