Querying the arrivals data for Chalfont & Latimer station (Metropolitan Line), only Northbound traffic is listed. (Only “inbound” provides a populated response)
This is consistent with the page: Chalfont & Latimer Underground Station - Transport for London which omits the Southbound (London-bound) movements
Is this a transient error or a gap in the data feed into the API?
I’m pretty sure this is because trains only appear in predictions once they’ve started their journey, so you at Chalfont & Latimer you would only get a train in arrivals once it has left either Amersham or Chesham.
This is a common issue near line termini - for example at Wimbledon Park you only see an eastbound train in arrivals once it has left Wimbledon. Last I heard of this, they were trying to find a solution for this issue, but no fix has been announced.
This happens because TfL decided to make their systems an “arrivals” one, rather than a “departures”. It is unable to show arrivals for a train until it has physically left the first station which means that the second station on the line often gets low levels of information.
As I went up to Chesham to have a look, there just isn’t any information at all at the station. In fact the whole section here down to Harrow-on-the-Hill where the Met shares the track with the National Rail is lacking in actual Met train information, both from TfL and from NRE Darwin.
I’m guessing that the 4-Line-Modernisation will eventually hook up with this section, but it’s not a high priority for London Transport as this section isn’t in London and it’s not really used that much (in the big scheme of things).
Thank you both for the responses. I did check, using the 09:09 departure from Chesham, two further stops up the line from here and it seems (no confirmation) that the southbound arrival information was available sometime after clearing the first station beyond journey commencement. Departures from Chesham would give a 8 to 9 minute window for update at Chalfont 7 Latimer, but the arrivals information for Rickmansworth was only available around 09:21, which would be consistent with departure from Chalfont & Latimer, with processing latency, or even Chroleywood (2 stops en route).
Seems I maybe need to use timetable data here until 4LM! 
There is also the Trackernet API, which if you fancy a bit of XML you can see the “raw” data for Chalfont & Latimer here
http://cloud.tfl.gov.uk/TrackerNet/PredictionDetailed/M/CLF
But, of course, it’s the same data as in the Unified API. And the National Rail data - also seen in the Darwin API is here
So, yes, the answer is… use the Timetable for now.
(If you want to try Trackernet and have the NRE three letter codes, this is the conversion …)
"KGX" => "KXX",
"ZEL" => "ELE",
"ZAT" => "ACT", "ZAG" => "AGR", "ZAD" => "ALD", "ZAE" => "ALE", "ZAP" => "ALP", "AMR" => "AME", "ZAN" => "ANG", "ZAR" => "ARC", "ZAL" => "ARL", "ZBB" => "BAR", "ZBW" => "BBB", "ZBQ" => "BCT", "ZBN" => "BDE",
"ZBD" => "BDS", "ZBZ" => "BEC", "ZBE" => "BER", "ZBG" => "BGR", "ZBI" => "BHL", "BHO" => "BHR", "BFR" => "BLF", "BET" => "BNG", "ZBA" => "BNK", "ZBU" => "BOR", "ZBM" => "BOS", "ZBP" => "BPK", "ZBS" => "BST",
"ZBT" => "BTX", "ZBO" => "BUR", "ZBR" => "BWR", "ZCK" => "CFS", "ZCF" => "CHF", "ZCG" => "CHG", "ZCI" => "CHP", "CFO" => "CLF", "ZCL" => "CLW", "ZCB" => "CNT", "ZCD" => "COL", "ZCV" => "COV", "ZCC" => "CPC",
"ZCP" => "CPK", "ZCN" => "CPN", "ZCA" => "CPS", "ZCR" => "CRD", "ZCO" => "CRX", "ZCT" => "CTN", "CLW" => "CWD", "ZCZ" => "CWF", "ZCW" => "CWR", "ZCY" => "CYL", "ZDB" => "DEB", "ZDE" => "DGE", "ZDH" => "DGH",
"ZDO" => "DHL", "ZEA" => "EAC", "EAL" => "EBY", "ZEC" => "ECM", "ZET" => "ECT", "ZED" => "EDG", "ZEF" => "EFY", "ZEH" => "EHM", "EPH" => "ELE", "ZEK" => "EMB", "ZEP" => "EPK", "ZEG" => "EPP", "ZPU" => "EPY",
"ZEB" => "ERB", "ZEM" => "ERD", "ZES" => "ESQ", "ZEE" => "ETE", "ZFD" => "FAR", "ZFB" => "FBY", "ZFA" => "FLP", "ZFR" => "FRD", "ZFC" => "FYC", "ZGG" => "GGR", "ZGA" => "GHL", "ZGE" => "GPK", "ZGP" => "GPS",
"ZGR" => "GRD", "ZGH" => "GRH", "ZGS" => "GST", "ZHT" => "HAI", "HRW" => "HAW", "ZHB" => "HBT", "HHY" => "HBY", "ZHU" => "HCH", "ZHD" => "HDN", "ZHG" => "HIG", "ZHL" => "HLN", "ZHA" => "HMD", "ZHM" => "HMP",
"ZHZ" => "HMS", "ZHC" => "HNC", "ZHN" => "HND", "ZHE" => "HNE", "ZHW" => "HNW", "ZHO" => "HOL", "ZHY" => "HPC", "ZHP" => "HPK", "HXX" => "HRC", "ZHR" => "HRD", "HWV" => "HRV", "HDN" => "HSD", "ZHS" => "HST",
"HAF" => "HRF", "ZHX" => "HTX", "ZIC" => "ICK", "ZKY" => "KBY", "ZKE" => "KEN", "KWG" => "KEW", "KNL" => "KGN", "ZKI" => "KIL", "ZKN" => "KNB", "ZKP" => "KPK", "STP" => "KXX", "ZLN" => "LAM", "ZLA" => "LAN",
"ZLY" => "LEY", "LBG" => "LON", "ZLC" => "LSQ", "ZLO" => "LTN", "ZLE" => "LYS", "ZMH" => "MAN", "ZMA" => "MAR", "ZMN" => "MCR", "ZMV" => "MDV", "MOG" => "MGT", "ZML" => "MHE", "ZME" => "MLE", "ZMR" => "MNR",
"ZMM" => "MON", "ZMO" => "MOR", "ZMP" => "MPK", "ZNA" => "NAC", "ZNN" => "NEA", "ZNE" => "NEL", "ZNP" => "NEP", "ZNF" => "NFD", "NGR" => "NGW", "ZNG" => "NHG", "ZNH" => "NHR", "ZNO" => "NHT", "ZND" => "NWD",
"ZNS" => "NWH", "NWB" => "NWM", "ZNK" => "NWP", "ZOA" => "OAK", "KPA" => "OLY", "ZOY" => "OST", "ZOV" => "OVL", "ZOC" => "OXC", "ZPE" => "PER", "ZPG" => "PGR", "ZPC" => "PIC", "ZPO" => "PIM", "ZPI" => "PIN",
"ZPS" => "PLW", "ZPK" => "PRY", "ZPB" => "PUT", "ZQB" => "QBY", "QPW" => "QPK", "ZQW" => "QWY", "ZRA" => "RCP", "ZRE" => "RED", "RIC" => "RKY", "ZRL" => "RLN", "ZRV" => "ROD", "ZRP" => "RPK", "ZRS" => "RSQ",
"ZRG" => "RUG", "ZRU" => "RUI", "ZRM" => "RUM", "SPB" => "SBC", "ZSE" => "SEL", "SRA" => "SFD", "SDS" => "SFS", "ZSA" => "SGT", "ZSH" => "SHL", "ZSX" => "SHR", "ZSS" => "SJP", "ZSJ" => "SJW", "ZSK" => "SKN",
"SOK" => "SKT", "ZSN" => "SNB", "SBP" => "SPK", "SRU" => "SRP", "ZSO" => "SSQ", "ZSM" => "STA", "ZSF" => "STB", "ZSG" => "STG", "ZSL" => "STK", "ZSY" => "STN", "ZSP" => "STP", "ZSC" => "SWC", "ZQS" => "SWF",
"ZSW" => "SWK", "ZSI" => "SWM", "ZTB" => "TBE", "ZTO" => "TBY", "ZTC" => "TCR", "ZTM" => "TEM", "ZTR" => "TGR", "ZTY" => "THB", "ZTH" => "THL", "ZTW" => "TOT", "ZTP" => "TPK", "ZTL" => "TPL", "TOM" => "TTH",
"ZUB" => "UPB", "ZUP" => "UPK", "ZUY" => "UPY", "ZUX" => "UXB", "VXH" => "VUX", "ZWE" => "WAC", "WHC" => "WAL", "ZWN" => "WAN", "ZWV" => "WAR", "ZWT" => "WAT", "WBP" => "WBT", "ZLW" => "WCL", "ZWC" => "WCT",
"WIM" => "WDN", "WMB" => "WEM", "ZWO" => "WFD", "ZWH" => "WFY", "ZWD" => "WGN", "WEH" => "WHM", "ZWX" => "WHR", "WIJ" => "WJN", "ZWK" => "WKN", "ZWG" => "WLG", "WAT" => "WLO", "ZWM" => "WMS", "ZWY" => "WPK",
"WRU" => "WRP", "ZWI" => "WSP", "ZWS" => "WST",