According to Wolfram Alpha, it's not a triangular number. 
http://www.wolframalpha.com/input/?i=is+2147129+a+triangular+number%3F
http://www.wolframalpha.com/input/?i=is+2147129+a+triangular+number%3F
"if the positive triangular root n of x is an integer, then x is the nth triangular number"
$ bc
((8 * 2147129 + 1) -1 )/2
8588516.00000000000000000000
I have to challenge your result here Prospero, sorry about that. Dug out my old mathbooks from the university to verify, it might still be wrong though so if someone else could verify it Please do.
http://i.imgur.com/d90yvj2.png
Yemmster: Your answer is not an integer. So does that mean its a dead end