There's just maybe a nuance. The service level agreements as you note are voluntary in the sense that the two parties are invited to negotiate them on a commercial basis. But it's important to note that the Fair Rail Freight Service Act and the changes to the CTA provide the shipper with the option to have a service level agreement, SLA, arbitrated if they can't negotiate one. So in that sense they're not fully voluntary. The railway can have an SLA imposed upon it through the arbitration process if a shipper and a railway aren't able to reach one through a negotiation.
The administrative monetary penalties, AMPs, that can be imposed by the CTA for non-compliance, are an attempt to focus the mind of railways and ensure that they will in fact comply with the obligations set out in the SLA.