For the retrofit of old Märklin all-current motors you need a permanent magnet.
Viessmann 8434 Marking Aid For Permanent Magnet Installation, H0
Permanent Magnet "between-the-rails" Delayed Uncoupler Kadee 321
Micro-Trains Line 310 Permanent Magnet Uncouplers (2) - N Gauge
ESU 51830 SwitchPilot 3 4-Fold Magnet Article Decoder NEW
ESU 41010 ~ New 2026 ~ Premium Foam Train Service Tray ~ With Magnet Recess