The Library of Congress provides public domain clips under the category of American Memory Collection.
There are sixteen major collections available, ranging from Coca Cola Advertising, clips from Edison Companies, animated films from 1900-1921, New York City films from 1898 to 1906 and Work and Leisure films from 1894 to 1915.
Depending on which category you choose, you can search by keyword, title index, index of subjects, alphabetical title list and chronological title list (by year). You can also search for a particular keyword by selecting all the categories or choose any of one or more of the categories.
I did a search for 'train' under New York City films and it turned out an impressive list of 54 videos, among which were train taking up mail bag (1903), freight train (1898) and Black Diamond Express (1897).
The Library provides video clips in three formats – MPEG, QuickTime and Real Media. Each minute of MPEG video takes up about 9 MB. As for QuickTime, each minute takes up about 6MB of storage space. Real Media is also offered but with a lower frame rate. You could choose this format to preview a clip in the streaming format before deciding on a download.
The MPEG and QuickTime videos are broken into segments if they exceed four minutes of running time. This is to keep the file size under 40 MB to make downloading easier for those with a slower Internet connection.
The Library makes it clear that it’s providing access to video clips for the purposes of research and education.
That means if you’re using it for non-profit purposes, you could use these video materials freely. The same would apply if you’re using the clips for home movies.
However, if you are going to use the clips for public distribution or reproduction, then it becomes your responsibility to locate the copyright owners or other rights holders if any and seek a written permission.
The library also assures that such clips are only in the public domain in the USA. If you’re going to use the clip in another country, it again becomes your responsibility to check whether you could use it without any copyright infringement.
If you’re using a public domain clip, the Library suggests the following credit line -: Library of Congress, Motion Picture, Broadcasting, and Recorded Sound Division.