Who is MRN? Do you mean Marin County? If so, 151.040 was set up many years ago by Marin County, not CDF, since there is no CDF ranger unit that covers Marin. 151.040 was supposed to be used as a coordination frequency with CDF for mutual aid but it has never been used as such in recent years since there are so many other fire mutual aid freqencies. As far as I know, CDF does not carry this freqency as part of their standard radio plan any longer either.
As Marin Co goes to trunked systems, the 151.040 will become Marin's CDF local. It has been talked about for a long time, but never used. Coming next fire season, probably. It will also be used for a variety of other things as well.
Yes, it is currently being used by Marin's DPW as Control 20. It has some of the best coverage for Marin outside of the new MERA system. It will be put into CDF radios as Marin's local. It may also be used for radio pagers for western Marin volunteer agencies, as MERA doesn't support standard radio pagers, and paging systems like Cooks (used by volunteers in eastern Marin) don't have consistent coverage in West Marin.
Other discussed options- use for air ambulances coming into Marin, a non MERA repeated dispatch channel (a benefit for owners of non-trunked scanners). This last idea was discussed for awhile, but seems to have been discarded.
There is another soon to be phased out Marin DPW channel, called Control 10, which is the 156.000 range, which may also be used for non-trunked needs.