Safety design on the railways concentrates on "Primary Safety", essentially the avoidance of accidents. The environment is rigorously controlled, with the result that accidents are very few. There are, of course, elements of "Secondary Safety", which aims to protect people from the effects of accidents; crush zones and other factors in stock design. The overall result of this is a well-protected passenger, with there being little benefit in having seat belts.
On the roads,the environment is virtually uncontrollable, so there has been much more emphasis on Secondary Safety, with seat belts, SIPS, airbags and the like. Even systems such as collision warning systems, which some claim to be Primary, are little more than a recognition that accidents are inevitable.
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...hey don't crash very often and when they do the forces involved tend to be so great that seatbelts are going to help much. ...
In fact, with those forces, it is more likely the seatbelt would cause more severe damage than being flung about the carriage.