Tazara train using the old wagons means cold nights where lots of clouding is required in July. At the moment there is a new train running on Fridays from Dar es Salaam, but the old train is still running from Dar on Tuesdays. Rumours says that there will come a new train within one year, which will make the journey more pleasant. When arriving in Kapiri Mposhi there are lots of taxis and minibuses that can bring to Lusaka. We payed 500 Kwacha for the whole family with taxi. You may expect to pay 60 kwacha going sharing minibus with others.
Traveling with NRZ (National Railway Zimbabwe) on 1st class is like going back 40 years, but still ok. There are no linen for sleeping so a silk sleeping bag and some extra cover like sarongs are preferable. In July you probable should use wool underwear to keep warm (temperature down to 2 C at night).