diff --git a/DCCQueue.cpp b/DCCQueue.cpp
new file mode 100644
index 0000000..bd9b282
--- /dev/null
+++ b/DCCQueue.cpp
@@ -0,0 +1,180 @@
+/*
+ *  © 2025 Chris Harlow
+ *  All rights reserved.
+ *  
+ *  This file is part of CommandStation-EX
+ *
+ *  This is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  It is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with CommandStation.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+
+#include "Arduino.h"
+#include "DCCQueue.h"
+#include "DCCWaveform.h"
+
+// create statics 
+DCCQueue* DCCQueue::lowPriorityQueue=new DCCQueue();
+DCCQueue* DCCQueue::highPriorityQueue=new DCCQueue();
+PENDING* DCCQueue::recycleList=nullptr;
+
+    DCCQueue::DCCQueue() {
+        head=nullptr;
+        tail=nullptr;
+    }
+    
+    void DCCQueue::addQueue(PENDING* p) {
+        if (tail) tail->next=p;
+        else head=p;
+        tail=p;
+        p->next=nullptr;          
+    }
+
+    void DCCQueue::jumpQueue(PENDING* p) {
+        p->next=head;
+        head=p;
+        if (!tail) tail=p; 
+    }
+
+    
+    void DCCQueue::recycle(PENDING* p) {
+        p->next=recycleList;
+        recycleList=p;
+    }
+    
+    // Packet joins end of low priority queue.
+    void DCCQueue::scheduleDCCPacket(byte* packet, byte length, byte repeats) {
+        lowPriorityQueue->addQueue(getSlot(NORMAL_PACKET,packet,length,repeats,0));
+    }
+    
+    // Packet replaces existing loco speed packet or joins end of high priority queue. 
+    
+    void DCCQueue::scheduleDCCSpeedPacket(byte* packet, byte length, byte repeats, uint16_t loco) {
+        for (auto p=highPriorityQueue->head;p;p=p->next) {
+            if (p->locoId==loco) {
+                // replace existing packet
+                memcpy(p->packet,packet,length);
+                p->packetLength=length;
+                p->packetRepeat=repeats;
+                return;
+            }
+        }
+        highPriorityQueue->addQueue(getSlot(NORMAL_PACKET,packet,length,repeats,loco));
+    }
+    
+    
+    // ESTOP -  
+    // any outstanding throttle packet for this loco (all if loco=0) discarded
+    // Packet joins start of queue,
+   
+    
+    void DCCQueue::scheduleEstopPacket(byte* packet, byte length, byte repeats,uint16_t loco) {
+        
+        
+        // kill any existing throttle packets for this loco
+        PENDING * previous=nullptr;
+        auto p=highPriorityQueue->head;
+        while(p) {
+            if (loco==0 || p->locoId==loco) {
+                // drop this packet from the highPriority   queue 
+                if (previous) previous->next=p->next;
+                else highPriorityQueue->head=p->next;
+                
+                recycle(p);  // recycle this slot
+
+                // address next packet
+                p=previous?previous->next : highPriorityQueue->head;
+            }
+            else {
+                previous=p;
+                p=p->next;
+            }
+        }
+        // add the estop packet to the start of the queue
+        highPriorityQueue->jumpQueue(getSlot(NORMAL_PACKET,packet,length,repeats,0));
+    }
+
+    // Accessory gate-On Packet joins end of queue as normal.
+    // When dequeued, packet is retained at start of queue 
+    // but modified to gate-off and given the delayed start.
+    // getNext will ignore this packet until the requested start time. 
+    void DCCQueue::scheduleAccOnOffPacket(byte* packet, byte length, byte repeats,int16_t delayms) {
+        auto p=getSlot(ACC_ON_PACKET,packet,length,repeats,0);
+        p->delayOff=delayms;
+        lowPriorityQueue->addQueue(p);
+    };
+
+    
+    // Obtain packet (fills packet, length and repeats) 
+    // returns 0 length if nothing in queue.
+
+    bool DCCQueue::scheduleNext() {
+        // check high priority queue first
+        if (!DCCWaveform::mainTrack.isReminderWindowOpen()) return false;
+        PENDING* previous=nullptr;
+        for (auto p=highPriorityQueue->head;p;p=p->next) {
+            // skip over pending ACC_OFF packets which are still delayed
+            if (p->type == ACC_OFF_PACKET && millis()<p->startTime) continue;
+            // use this slot
+            DCCWaveform::mainTrack.schedulePacket(p->packet,p->packetLength,p->packetRepeat);
+            // remove this slot from the queue 
+            if (previous) previous->next=p->next;
+            else highPriorityQueue->head=p->next;
+            
+            // and recycle it.
+            recycle(p);     
+            return true;
+        }
+        
+        // No high priopity packets found, check low priority queue
+        auto p=lowPriorityQueue->head;
+        if (!p) return false;  // nothing in queues
+        
+        // schedule first packet in queue 
+        DCCWaveform::mainTrack.schedulePacket(p->packet,p->packetLength,p->packetRepeat);
+
+        // remove from queue
+        lowPriorityQueue->head=p->next;
+        if (!lowPriorityQueue->head) lowPriorityQueue->tail=nullptr;
+        
+        if (p->type == ACC_ON_PACKET) {
+                // convert to a delayed off packet and jump the high priority queue
+                p->type= ACC_OFF_PACKET;
+                p->packet[1]  &= ~0x08; // set C to 0 (gate off) 
+                p->startTime=millis()+p->delayOff;
+                highPriorityQueue->jumpQueue(p);
+                }
+        else recycle(p);  // recycle this slot
+        return true;   
+    }
+    
+    // obtain and initialise slot for a PENDING.
+    PENDING*  DCCQueue::getSlot(PendingType type, byte* packet, byte length, byte repeats,uint16_t loco) {
+        PENDING * p; 
+        if (recycleList) {
+            p=recycleList;
+            recycleList=p->next;
+        }
+        else { 
+            p=new PENDING; // need a queue entry 
+        }
+        p->next=nullptr;
+        p->type=type;
+        p->packetLength=length;
+        p->packetRepeat=repeats; 
+        memcpy((void*)p->packet,packet,length);
+        p->locoId=loco;
+        return p; 
+    }
+
+    
diff --git a/DCCQueue.h b/DCCQueue.h
new file mode 100644
index 0000000..8958d6b
--- /dev/null
+++ b/DCCQueue.h
@@ -0,0 +1,84 @@
+/*
+ *  © 2025 Chris Harlow
+ *  All rights reserved.
+ *  
+ *  This file is part of CommandStation-EX
+ *
+ *  This is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  It is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with CommandStation.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef DCCQueue_h
+#define DCCQueue_h
+#include "Arduino.h"
+#include "DCCWaveform.h"
+
+enum PendingType:byte {NORMAL_PACKET,ACC_ON_PACKET,ACC_OFF_PACKET,DEAD_PACKET};
+  struct PENDING {
+      PENDING* next; 
+      PendingType type;
+      byte packetLength;
+      byte packetRepeat;
+      byte packet[MAX_PACKET_SIZE];
+      
+      union { // use depends on packet type
+        uint16_t locoId;    // NORMAL_PACKET .. only set >0 for speed change packets 
+                            // so they can be easily discarded if an estop jumps the queue.
+        uint16_t delayOff;  // ACC_ON_PACKET delay to apply between on/off
+        uint32_t startTime; // ACC_OFF_PACKET time (mS) to transmit 
+      };
+  };
+  
+class DCCQueue {
+  public:
+    
+    
+    // Non-speed packets are queued in the main queue
+    static void scheduleDCCPacket(byte* packet, byte length, byte repeats);
+
+    // Speed packets are queued in the high priority queue
+    static void scheduleDCCSpeedPacket(byte* packet, byte length, byte repeats, uint16_t loco);
+    
+    // ESTOP packets jump the high priority queue and discard any outstanding throttle packets for this loco  
+    static void scheduleEstopPacket(byte* packet, byte length, byte repeats,uint16_t loco);
+
+    // Accessory gate-On Packet joins end of main queue as normal. 
+    // When dequeued, packet is modified to gate-off and given the delayed start in the high priority queue. 
+    // getNext will ignore this packet until the requested start time.
+    static void scheduleAccOnOffPacket(byte* packet, byte length, byte repeats,int16_t delayms);
+
+  
+    // Schedules a main track packet from the queues if none pending.  
+    // returns true if a packet was scheduled.
+    static bool scheduleNext(); 
+
+  private:
+  
+  // statics to manage high and low priority queues and recycleing of PENDINGs
+    static PENDING* recycleList;
+    static DCCQueue* highPriorityQueue;
+    static DCCQueue* lowPriorityQueue;
+
+    DCCQueue();
+    
+    PENDING*  head;
+    PENDING * tail;
+    
+    // obtain and initialise slot for a PENDING. 
+    static PENDING*  getSlot(PendingType type, byte* packet, byte length, byte repeats, uint16_t loco);
+    static void recycle(PENDING* p);
+    void addQueue(PENDING * p);
+    void jumpQueue(PENDING * p);
+  
+};
+#endif
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