Getting a seat on a crowded train & Offering your seat
Generally speaking, Japanese are very oriented on getting a seat on the train, as this is an opportunity to get some much-needed sleep! So, what will more often than not happen is people will generally try to avoid giving up a seat for someone who needs it.
Offering a seat although it is done by good-mannered people, often the Japanese will offer to a fellow Japanese but the upshot will be that the standing person will decline even if their need is greater than the seated ones.
The seated person will only do it to clear their conscience.
Therefore, if you see someone who is in need of a seat, (pregnant, unstable on their feet etc); I would normally just offer the seat, but don't engage in conversation as Japanese will normally decline and I have found it is just better to not make a fuss about it.