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