Champaran HumSafar Express : इन अजनबी सी राहों में, जो तू मेरा हमसफ़र हो जाये, बीत जाए पल भर में ये वक़्त, और हसीन सफ़र हो जाये. - Piyush Singh

WAP7 has greater horse power but max speed of 140kmph and usually runs trains @120-130
WAP5 has lesser horse power but greater max speed of 160kmph.
WAP5 are usually given to short trains and trains that don't have to climb up gradients. It is for this reason that Howrah/Sealdah Rajdhanis (crossing Chotanagpur hills) and both Kalka Shatabdis (going uphill from Chandigarh to Kalka) are usually given WAP7. Although some days you may find these trains with a WAP5, yet WAP7 gives better performance on these routes.
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was told to me by a Chittaranjan Loco Works (CLW) engineer who was my co-passenger in the First AC coupe from Asansol, while returning from Calcutta by 2313 Sealdah Rajdhani in 2008.
And yes, 2001/2002 Bhopal Shatabdi regularly has a WAP5 for touching 150kmph between Delhi and Agra. Since, this train passes through hilly terrain between Gwalior and Jhansi, it has only 8 CC & 1 EC coaches and is a comparatively shorter Shatabdi than the others.
Hope this helps. Thanks for your patience :-)