update (28 June 2019): Tour company Namibia Reservations has released more information about the famous "Desert Express". The luxury train will be travelling between Windhoek and Swakopmund on the following weekends in 2019: 26 July, 09 & 23 August, 06 & 27 September, 11 & 25 October, 15 & 19 November and 13 December. One way tickets (Windhoek > Swakopmund) cost N$4400/person (GBP245 | €273 | USD310) sharing; return tickets (Windhoek > Swakopmund > Windhoek) cost N$6160/person sharing (GBP343 | €382 | USD435). Namibian and SADC rates are available on request.
According to Namibian specialist business website Namibia Economist, the "Desert Express", a weekly tourist-orientated train service between Windhoek and Swakopmund, will be operational again from the end of June 2019. Further details, such as the train schedule and train fares, are not known yet, but will be published at a later stage.
The "Desert Express" service was launched by TransNamib, the state-owned railway enterprise of Namibia, in 1998 and got (temporarily) suspended in 2015 due to a lack of maintenance.