Being an MP: public service or a career?

A correspondent writes:

I was struck by this headline in the Independent yesterday. It takes for granted what many of us suspect: that many MP’s (and councillors) see their jobs in terms of careers with opportunities for self advancement rather than the privilege of representing their constituents in public service. To vote against the whip is to forfeit your “career” chances.
I wonder why, in the circumstances, so many voted against HS2?  Are they unusually brave and principled or just more frightened by the May 2015 ballot box? If this is a power game, we the voters on issues like this, outnumber the whips.
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Indy MP Careers