Shafaqna (International Shia News Agency) – The Grand Ayatollah Sistani answered some questions regarding prostration. According to Shafaqna the questions and answers are as follows:
Question: How should we say our salat in trains and cars? Is it necessary to do prostration (sajda) on something or is it not necessary, in that would bending of the neck be sufficient?
Answer: It is obligatory to say salat in the usual way where possible. So, one should face the qiblah in all stages of the salat; if not, at least while saying the opening takbir. Otherwise the condition of facing the qiblah will be dropped. Similarly, if it is possible to do the bowing (ruku’) and prostration (sajdah) normally (e.g., in the aisle of the bus or the train), those parts of salat should be done normally. But if it is not possible, then one should try to bow normally for ruku’ and sajdah [for example, from a sitting position on the seat or the berth of the train].
For sajdah, one has to put the forehead on an item on which sajdah is valid, even if by lifting that item to the forehead. If bowing normally is not possible, one should just indicate by bending the neck [halfway for ruku and fully for sajdah].
2Question: Is it alright to do sajdah on concrete or on mosaic?
Answer: Yes, it is alright.
3Question: Some prayer-mats are made of synthetic material; is it permissible to do sajdah on them?
Answer: Sajdah on such items is not sufficient.
4Question: A reciter of the Holy Qur’an recites a verse of wajib sajdah, on hearing it from a cassette player, is it obligatory on us to do sajdah in this case?
Answer: It is not obligatory.
5Question: Is it permissible to do sajdah on writing paper and on paper tissues, especially, if is not known whether or not the raw material they are made of was from items on which sajdah is valid?
Answer: It is not permissible to do sajdah on paper tissues, only after ascertaining that they have been made from items on which sajdah is allowed; it is permissible to do sajdah on paper if it is made from material on which sajdah is allowed or from cotton or flax.
Source: Ayatollah Sistani