Union representatives met ScotRail management, Transport Minister Derek MacKay, and Transport Scotland officials over the issue.
The RMT said about 40% of the ScotRail fleet should be fitted with controlled emission tanks by April 2016.
The remainder are due to be fully fitted by December 2017.
RMT General Secretary Mick Cash said: "The fact the programme for the elimination of the filthy, disgusting and dangerous practice of dumping sewage on our railways  is further advanced in Scotland is 100% down to the hard work and campaigning over several years by representatives, members and officials of RMT.
"Network Rail in Scotland are also in the process of arranging briefings to frontline maintenance staff and offering vaccinations to reduce the risk to staff from untreated human excrement which is another important development resulting from the RMT campaign.
"If real targets can be laid down for ending the scandal of sewage on our railways in Scotland then they can be achieved across the whole network."
The date for the elimination of trains dumping sewage on Scotland's rail tracks has been brought forward after a campaign, the RMT union has said.