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Along a tunnel this train races;

Stops not at stations, leaps the spaces,

Lost and sometimes found again

Surprised to find me here, as when

You'd never me, ivory orb

In which, though you're not water, you're absorbed.

'I love trains, but they're awfully fast. I wouldn't want to race one.'
'Well there's no point in waiting at a station for this one.'

Riddle Answer A Thought

Riddle Explanation
along a tunnel this train races - a train of thoughts.

lost and sometimes found again - to lose one's train of thought.

surprised to find me here, as when you'd never me - never thought I'd see you here!

ivory orb in which, though you're not water, you're absorbed - to be absorbed in thought.