==Corrupt PDF Previews==
Only some sizes of some pages of PDF previews are corrupt, e.g. https://commons.wikimedia.org/w/index.php?title=File%3ATrolley-Trips-Through-New-England.pdf&page=17 shows only gray lines in the default preview while other sizes render OK. Also see images associated with the "next page" and "previous page" links—some there seem to render only the first layer. When using linux's `pdfimages` tool on the original PDF the page in question separates out to 3 layers--a pbm (monochrome), and couple pnm layers. PDF Displays fine directly in Chrome/Firefox and also XPdf, albeit slowly… seems to be thumbnailer/pdf renderer issue, perhaps.
Purged cache for page this did not solve.
===Corrupt===
* https://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Trolley-Trips-Through-New-England.pdf/page17-389px-Trolley-Trips-Through-New-England.pdf.jpg
===Good===
* https://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Trolley-Trips-Through-New-England.pdf/page17-635px-Trolley-Trips-Through-New-England.pdf.jpg
* https://upload.wikimedia.org/wikipedia/commons/thumb/f/fb/Trolley-Trips-Through-New-England.pdf/page17-155px-Trolley-Trips-Through-New-England.pdf.jpg