Platform-Bob states that both red and blue lightning occurred at the same time. Whereas, Train-Bob states that the red lightning occurred before the blue. Why?

• As seen from the platform, Train-Bob is moving towards the red lightning and away from the blue lightning. So the bubble from red lightning reaches Train-Bob before the one from blue.
• The instant when each bubble reaches Train-Bob in the platform reference frame, is the same instant when its doppleganger reaches Train-Bob on the train reference frame.
• But when Train-Bob receives the light signal in the train reference frame, it is old news. It is from way back when the events were occurring only half a train length away.
• He double-checks this by observing the marks left by the lightning bolts on either end of the train.
• And so, per Train-Bob's own observation (path of his own light bubbles), light from each lightning strike has traveled the same distance, to reach him at different times.
• Based on this, and the postulate that the speed of light is constant in either direction, Train-Bob concludes that, indeed, the red lightning occurred before the blue!

Therefore, we see that a time interval between the two events as measured from the platform, may not be the same when measured from the moving train. Time, like distance, is relative to the observer's frame of reference!