A negative look ahead assertion at the start of the line (not a look back) is what you want:

use strict;
use warnings;

while (<DATA>) {
  print "Matched: $_" if /^(?!help).*?train times (?!to\b|from\b)\w/;
}
__DATA__
help train times from xyz
load train times including xyz
help train times including xyz
book train times at 1234
[download]

Prints:

Matched: load train times including xyz
Matched: book train times at 1234
[download]