In high res format, so they're zoom-able to see the details you need for a really great build... Unfortunately, you can't save them, or at least not directly.
http://cdm15323.contentdm.oclc.org/cdm4/results.php?CISOOP1=any&CISOFIELD1=CISOSEARCHALL&CISOROOT=/p15323coll6&CISOBOX1=Railroad
cool!
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