Re: Luger S/42 G
The following information comes from the reference book "German Belt Buckles" by Thomas Reid:
The German Railway Corporation (Deutsche Reichbahn Gesellshaft [DRBG]), founded in 1835, did not adopt a military style buckle until the mid 1920s. In 1933 a variety was introduced which featured a stylized Weimar eagle. This buckle was used, along with the earlier version, from 1933-1935.
Since your railway buckle has a swastika, it was of the pattern introduced after 1935, to reflect the Nazi government.
As for value, I have not been able to find any similar buckles which are being offered for sale, so it may very possibly have a value anywhere from $100 to $500.