elroy said: Ok, in that case you need a preposition, because تهبط is not a transitive verb: سكة حديد تصعد وتهبط على المنحدرات. Click to expand...

على , but with the meaning of descending or going down it is transitive and takes a direct object.

This verb is similar to equivalent English verbs in that it can be transitive and intransive. You don't descend on slopes' or 'drop down on slopes', but rather 'descend slopes' or drop down slopes'. With the meaning of land on or set down it is intransitive and you use the preposition