If you look at the TxWARN P25 system (linked in my earlier post), you'll see a group pf talkgroup ID's under the heading Metropolitan Transit Authority of Harris County Talkgroups.
The talkgroups for rail operations, including the Red, Green, and Purple lines, are in that section. While I'm not in Houston, I believe that the site you'd need to download (or program) would be the Houston General Services Simulcast, 03-005. Those talkgroups are designated with a D in the mode column, so are P25 (digital) Phase I. The Houston public safety talkgroups, for Police and Fire, are mostly Phase II, as shown by the T in the mode column.
Negative on the GGSL, you want Zone 1 (RFSS 1) site 52 (Harris County Simulcast).
Here is a quick view of the light rail TG's...
2347 92b D MetRail1 Ops MetroRail Ch. 1 Red Line Operations Control Primary Transportation
2348 92c D MetRail2 Ops MetroRail Ch. 2 Red Line Operations Control Alternate Transportation
2349 92d D MetRail3Yard MetroRail Ch. 3 Yard Transportation
2350 92e D MetRail4Talk MetroRail Ch. 4 Talk-Around Transportation
2351 92f D MetRail5 Mnt MetroRail Ch. 5 Maintenance Transportation
2352 930 D MetroRail 6 MetroRail Ch. 6 Transportation
375 177 D MetroRailDiv MetroRail Division Transportation
1982 7be D MetroRailGr1 MetroRail Green Line 1 Transportation
1978 7ba D MetroRailGr2 MetroRail Green Line 2 Transportation
1952 7a0 D MetroRailHrs MetroRail Harrisburg Transportation
946 3b2 D MetroRailMWP MetroRail Maintenance of Way - Purple Line Transportation
1980 7bc D MetroRailMWR MetroRail Maintenance of Way - Red Line Transportation
1976 7b8 D MetroRailPu1 MetroRail Purple Line 1 Transportation
1984 7c0 D MetroRailPu2 MetroRail Purple Line 2 Transportation
1986 7c2 D MetroRailTra MetroRail Traffic T