Railway postcards were one of the most popular items in quarter of the twentieth century. However, today, it has become a sub-category of postcard collecting because:
The golden age of it is passed! In fact, travelling by train used to be the most common way of transportation in early 20th century. It was very common those days for
a passenger to send postcards while traveling on a train to be either in contact or send family and friends souvenirs.
They were valuable and people used to keep them lifetime rather than throwing them away or sell them in a garage sale!
Taking photos and  finding a postcard printer was difficult and unique in old days comparing to today that you can find them even online!