All railway signs imply a maximum speed, which defaults to 160 km/h if nothing is set explicitly.
Preciesely, minimum speed, one way and end of choose signs imply such a speed limit, which does not make a lot of sense to me.
Such signs should by default not imply any speed limit. If desired, a speedlimit can explicitly be set.
Minimum speed signs as well as end of choose signs do not exist as such in the real-world and it's nothing the train actually needs to take care about. It's an information about the tracks which you can pass to the virtual simutrans train dispatcher by using those signs.
The same for oneway signals. Such do not exist in the real-world. The dispatcher will handle such things. A train driver does not need to take care about this, so a slowdown doesn't make sense here as well.