icon-image properties that are dynamically created are not supported

[DerDakon]

Try this:

node|z14-[railway=signal]["railway:signal:speed_limit"=lf7]["railway:signal:speed_limit:speed"]
{
icon-image: eval(concat("icons/de-lf7-", tag("railway:signal:speed_limit:speed"), "-sign-32.png"));
}