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RTP is exactly what it says - the percentage of trains within the time slot.
PPM contains an element of weighting, so that different TOCs with very different routes/traction/services can be compared with each other. It also takes into account Failures and Cancellations.
The significance of the PPM is that is used as an indication of how well a TOC is performing in terms of its franchise commitment.
HTH
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"Everything was built by men in overalls and destroyed by men in suits" Fred Dibnah
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