budget dcc youtube --> https://www.youtube.com/watch?v=9-TJmxLq8jw
ble $2.95 - https://www.seeedstudio.com/HM-BT4502-Bluetooth-Low-Energy-BLE-Pass-through-Module-p-4440.html
booster $4.95 - https://www.pololu.com/product/2136
booster can be wired parallel to increase drive current see datasheet

pinout connections

bluetooth
ble vdd - pico pin 36 3.3v
ble gnd - pico any ground pins
ble rxd - pico pin 6 gp4
ble txd - pico pin 7 gp5

booster drv8801
drv8801 vdd         - pico pin 36 3.3v
drv8801 gnd         - pico any ground pins
drv8801 pwm(enable) - pico pin 36 3.3v
drv8801 dir(phase)  - pico pin 20 gp15
drv8801 vmm         - power supply positive 12v - 15v
drv8801 gnd         - power supply negative
drv8801 out +       - track rail (can be interchange with out -)
drv8801 out -       - track rail (can be interchange with out +)

#############pico micropython code
#dcc demo
from machine import UART, Pin
from micropython import const
import time, rp2, array, struct

l0=Pin(25,Pin.OUT)
l1=Pin(16,Pin.OUT)
l2=Pin(17,Pin.OUT)
uart=UART(1,115200)

l0.on()
time.sleep(2)
l0.off()
time.sleep(2)
l0.on()

ADDRESS = const(20) #santa fe
speed = 0
isForward = True
data = 0
dir = 0x60
packet = bytearray(b'\xff\xff\xfe\xff\xff\xff\xff\xff')
idle  =  bytearray(b'\xff\xfd\xfe\x00\x7f\xc0\x00\x00')

def init():
    global packet
    packet = bytearray(b'\xff\xff\xfe\xff\xff\xff\xff\xff')

def assemble(addr, d):
    global packet
    checksum = addr ^ d
    packet[3] = addr
    packet[4] = d >> 1
    packet[5] = checksum >> 2
    if not((d >> 7) & 1):
        packet[5] |= 1 << 7
    temp = checksum << 6
    packet[6] = temp | 0x3f


@rp2.asm_pio(set_init=rp2.PIO.OUT_LOW,
             out_shiftdir=rp2.PIO.SHIFT_LEFT, autopull=True)
def dcc():
    label("bitloop")
    set(pins,1) [20] 
    out(x,1)                  
    jmp(not_x,"do_zero")       
    set(pins,0) [21] 
    jmp("bitloop")             
    label("do_zero")
    nop() [16]
    set(pins,0) [30]
    nop() [8]  

# freq = 400_000 2.5us clock cycle
sm = rp2.StateMachine(0, dcc, freq=400000, set_base=Pin(15))
sm.active(1)

while 1:
    if uart.any() > 0:
        c = uart.read(1)
        cc = c.decode('UTF-8')
        if cc == '$':
            
            if uart.any() > 0:
                d = uart.read(1)
                dd = d.decode('UTF-8')
                
                if dd == 'd': #direction
                    isForward = not isForward
                    if isForward:
                        data = 0x61
                        dir = 0x60
                    else:
                        data = 0x41
                        dir = 0x40
                        
                if dd == 's': data = 0x41       #stop             
                if dd == 'F': data = dir + 0x0f #FAST
                if dd == 'M': data = dir + 0x09 #MEDIUM
                if dd == 'S': data = dir + 0x05 #SLOW
        
                #print(hex(data))
        
                init() #reset packet
                assemble(ADDRESS,data)
                w1,w2=struct.unpack('>II',packet)
                sm.put(w1)
                sm.put(w2)
                time.sleep_ms(5) #nmra specs
        
    else: #send idle packets
        w1,w2=struct.unpack('>II',idle)
        sm.put(w1)
        sm.put(w2)
        time.sleep_ms(5) #nmra specs
        
// flutter files
// make sure change build.gradle to minsdk to atleast 21
// part of pubspec.yaml
dependencies:
  flutter:
    sdk: flutter
  get:
  flutter_reactive_ble:
  group_button:
  get_storage:
/////////////////////filename: main.dart

import './listing.dart';
import 'package:group_button/group_button.dart';
import './blecontroller.dart';
import './controller.dart';

void main() async {
await GetStorage.init();
runApp(MyApp());}

class MyApp extends StatelessWidget {

final appdata = GetStorage();
TextStyle myStyle = TextStyle(fontSize: 20);  
final BleController ble = Get.put(BleController());
final Controller c = Get.put(Controller());

void processButton(int i){
switch(i){
case 0:{c.toggleDirection();}break;
case 1:{c.setSpeed(i);}break;
case 2:{c.setSpeed(i);}break;
case 3:{c.setSpeed(i);}break;
case 4:{c.setSpeed(i);}break;}
ble.send(i);}

@override Widget build(BuildContext context){
appdata.writeIfNull('darkmode',false);
return SimpleBuilder(builder:(_){
bool isDarkMode = appdata.read('darkmode');
return GetMaterialApp(
debugShowCheckedModeBanner:false,
theme: isDarkMode ? ThemeData.dark() : ThemeData.light(),

home: Scaffold(

appBar:AppBar(title: Text('DCC BLE'),
actions:[Switch(value:isDarkMode,
onChanged:(value)=>appdata.write('darkmode',value))]),

body: Column(children: <Widget> [

SizedBox(height:50),

Obx(() => Text('${ble.status}', style:myStyle)),
    
SizedBox(height:40),

Row(
mainAxisAlignment:MainAxisAlignment.center,
children:[
Text('direction: ',style:myStyle),
Obx(()=>Icon(c.direction.value == 'forward' ?Icons.arrow_back_rounded : Icons.arrow_forward_rounded)),
Text('speed: ',style:myStyle),
Obx(()=>Text('${c.speed.value}',style:myStyle))]),

SizedBox(height:40),
   
GroupButton(
isRadio: true,
spacing: 10,
onSelected: (index, isSelected) => processButton(index),
buttons: ["direction","fast", "medium", "slow", "stop"],
borderRadius: BorderRadius.circular(5.0),),

SizedBox(height:40),
  
Obx(() => Container(
child: ble.status != 'connected!'
? ElevatedButton(onPressed: ble.connect,
 child: Text('connect',
 style:myStyle))
: null))
])),);});}}

///////////////////////// filename: blecontroller.dart

import './listing.dart';
import 'package:flutter_reactive_ble/flutter_reactive_ble.dart';
import 'dart:async';

class BleController {
final frb = FlutterReactiveBle();
late StreamSubscription <ConnectionStateUpdate> c;
late QualifiedCharacteristic tx;
RxString status = 'not connected'.obs;
List<int> values = [0x24,0,0x00];

void send(int i) async {
switch(i){
case 0: {values[1]=0x64;}break;
case 1: {values[1]=0x46;}break;
case 2: {values[1]=0x4d;}break;
case 3: {values[1]=0x53;}break;
case 4: {values[1]=0x73;}break;}
await frb.writeCharacteristicWithoutResponse(tx,value:values);}

void connect() async {
status.value = 'connecting...';
c = frb.connectToDevice(id: '62:00:A1:1C:46:54').listen((state){
if (state.connectionState == DeviceConnectionState.connected){         
status.value = 'connected!';
  
tx = QualifiedCharacteristic(
serviceId: Uuid.parse("ffe0"),            
characteristicId: Uuid.parse("ffe9"), 
deviceId:'62:00:A1:1C:46:54');}});}}

////////////////// filename: listing.dart

export 'package:flutter/material.dart';
export 'package:get/get.dart';
export 'package:get_storage/get_storage.dart';

/////////////////filename: controller.dart

import 'package:flutter/material.dart';
import 'package:get/get.dart';

class Controller{
var direction = 'forward'.obs;
var speed = 'stop'.obs;

toggleDirection(){
if (direction.value == 'forward')
direction.value = 'backward';
else
direction.value = 'forward';} 

setSpeed(int s){
switch(s){
case 1:{speed.value = 'fast';}break;
case 2:{speed.value = 'medium';}break;
case 3:{speed.value = 'slow';}break;
case 4:{speed.value = 'stop';}break;
}}}