Question 903977
You can think of this as one train
standing still and the other one 
moving at the sum of their speeds
Let {{{ s }}} = the speed of the slower train
in km/hr
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{{{ 500 = ( s + s + 50 )*2 }}}
{{{ 500 = 4s + 100 }}}
{{{ 4s = 400 }}}
{{{ s = 100 }}}
{{{ s + 50 = 150 }}}
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Toronto to Montreal train:
{{{ d[1] = 100*2 }}}
{{{ d[1] = 200 }}}
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Montreal to Toronto train:
{{{ d[2] = 150*2 }}}
{{{ d[2] = 300 }}}
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The trains pass 200 mi from Toronto