Can a fly stop a train?

[Barnacle_Bill]

Probably an old chestnut, but it's been worrying me since schooldays - and that's a long time.

A train is heading down the track and a fly is travelling up it. The two collide. The fly's velocity has changed from positive to negative (or vice versa!) so at some instant in time it must have been zero. At that instant it was in contact with the train so the train's velocity must also have been zero. So for that instant the fly stopped the train. I don't think so, but where's the flaw in the logic?

The only thing I can think of is that the fly's velocity has INSTANTLY changed without going through zero, but that doesn't sound very satisfactory. Can anyone finally put me out of my misery? (I hope calculus is not involved)

[factor-fiction]

I support vascop on this one.
A number of people have argued that if two things are touching then both must have the same speed. Does that apply to dust particles in the air as well as flies? So when the train is stationery how does it ever start moving?

[vascop]

Thanks for your support, factor! I think the others will say, as they already have, that they are not saying that the train stops but only the tiny part in contact with the ball.
However this is also wrong in my opinion.

[beso]

Did the stain stoppers accept they were wrong or give up?

Except for a rather meek post by RJUKL nobody has disputed my detailed atomic level explanation. I thought it pretty much left the other side nowhere to go.

[Cheryl_Tweedy]

Or even a flying drop