In American English, tunnel is inflected with one l—tunneled and tunneling. This has been the case since the early 20th century, when many -el-ending verbs went through a similar transition in American English.
Outside North America, the older forms, tunnelled and tunnelling, still prevail by a large margin, and the American forms are likely to be seen as misspellings.
Both the American spellings and the non-U.S. spellings are common in published Canadian writing from this century. Of course, many Canadians would say Canadian English generally follows British English in its verb inflections, but this is not consistently borne out in practice, at least when it comes to tunnel and other –el verbs.